Posit AI Weblog: torch time collection, take three: Sequence-to-sequence prediction
Right this moment, we proceed our exploration of multi-step time-series forecasting with torch. This put up is the third in a collection.
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Initially, we coated fundamentals of recurrent neural networks (RNNs), and educated a mannequin to foretell the very subsequent worth in a sequence. We additionally discovered we may forecast fairly a number of steps forward by feeding again particular person predictions in a loop.
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Subsequent, we constructed a mannequin “natively” for multi-step prediction. A small multi-layer-perceptron (MLP) was used to undertaking RNN output to a number of time factors sooner or later.
Of each approaches, the latter was the extra profitable. However conceptually, it has an unsatisfying contact to it: When the MLP extrapolates and generates output for, say, ten consecutive time limits, there isn’t a causal relation between these. (Think about a climate forecast for ten days that by no means obtained up to date.)
Now, we’d wish to attempt one thing extra intuitively interesting. The enter is a sequence; the output is a sequence. In pure language processing (NLP), this sort of activity is quite common: It’s precisely the sort of scenario we see with machine translation or summarization.
Fairly fittingly, the sorts of fashions employed to those ends are named sequence-to-sequence fashions (usually abbreviated seq2seq). In a nutshell, they cut up up the duty into two elements: an encoding and a decoding half. The previous is completed simply as soon as per input-target pair. The latter is completed in a loop, as in our first attempt. However the decoder has extra data at its disposal: At every iteration, its processing relies on the earlier prediction in addition to earlier state. That earlier state would be the encoder’s when a loop is began, and its personal ever thereafter.
Earlier than discussing the mannequin intimately, we have to adapt our knowledge enter mechanism.
We proceed working with vic_elec , offered by tsibbledata.
Once more, the dataset definition within the present put up seems to be a bit totally different from the way in which it did earlier than; it’s the form of the goal that differs. This time, y equals x, shifted to the left by one.
The explanation we do that is owed to the way in which we’re going to practice the community. With seq2seq, individuals usually use a way referred to as “instructor forcing” the place, as a substitute of feeding again its personal prediction into the decoder module, you go it the worth it ought to have predicted. To be clear, that is accomplished throughout coaching solely, and to a configurable diploma.
library(torch)
library(tidyverse)
library(tsibble)
library(tsibbledata)
library(lubridate)
library(fable)
library(zeallot)
n_timesteps <- 7 * 24 * 2
n_forecast <- n_timesteps
vic_elec_get_year <- perform(12 months, month = NULL) {
vic_elec %>%
filter(12 months(Date) == 12 months, month(Date) == if (is.null(month)) month(Date) else month) %>%
as_tibble() %>%
choose(Demand)
}
elec_train <- vic_elec_get_year(2012) %>% as.matrix()
elec_valid <- vic_elec_get_year(2013) %>% as.matrix()
elec_test <- vic_elec_get_year(2014, 1) %>% as.matrix()
train_mean <- imply(elec_train)
train_sd <- sd(elec_train)
elec_dataset <- dataset(
identify = "elec_dataset",
initialize = perform(x, n_timesteps, sample_frac = 1) {
self$n_timesteps <- n_timesteps
self$x <- torch_tensor((x - train_mean) / train_sd)
n <- size(self$x) - self$n_timesteps - 1
self$begins <- kind(pattern.int(
n = n,
measurement = n * sample_frac
))
},
.getitem = perform(i) {
begin <- self$begins[i]
finish <- begin + self$n_timesteps - 1
lag <- 1
record(
x = self$x[start:end],
y = self$x[(start+lag):(end+lag)]$squeeze(2)
)
},
.size = perform() {
size(self$begins)
}
)Dataset in addition to dataloader instantations then can proceed as earlier than.
batch_size <- 32
train_ds <- elec_dataset(elec_train, n_timesteps, sample_frac = 0.5)
train_dl <- train_ds %>% dataloader(batch_size = batch_size, shuffle = TRUE)
valid_ds <- elec_dataset(elec_valid, n_timesteps, sample_frac = 0.5)
valid_dl <- valid_ds %>% dataloader(batch_size = batch_size)
test_ds <- elec_dataset(elec_test, n_timesteps)
test_dl <- test_ds %>% dataloader(batch_size = 1)Technically, the mannequin consists of three modules: the aforementioned encoder and decoder, and the seq2seq module that orchestrates them.
Encoder
The encoder takes its enter and runs it by an RNN. Of the 2 issues returned by a recurrent neural community, outputs and state, thus far we’ve solely been utilizing output. This time, we do the alternative: We throw away the outputs, and solely return the state.
If the RNN in query is a GRU (and assuming that of the outputs, we take simply the ultimate time step, which is what we’ve been doing all through), there actually is not any distinction: The ultimate state equals the ultimate output. If it’s an LSTM, nonetheless, there’s a second sort of state, the “cell state”. In that case, returning the state as a substitute of the ultimate output will carry extra data.
encoder_module <- nn_module(
initialize = perform(kind, input_size, hidden_size, num_layers = 1, dropout = 0) {
self$kind <- kind
self$rnn <- if (self$kind == "gru") {
nn_gru(
input_size = input_size,
hidden_size = hidden_size,
num_layers = num_layers,
dropout = dropout,
batch_first = TRUE
)
} else {
nn_lstm(
input_size = input_size,
hidden_size = hidden_size,
num_layers = num_layers,
dropout = dropout,
batch_first = TRUE
)
}
},
ahead = perform(x) {
x <- self$rnn(x)
# return final states for all layers
# per layer, a single tensor for GRU, an inventory of two tensors for LSTM
x <- x[[2]]
x
}
)Decoder
Within the decoder, similar to within the encoder, the primary element is an RNN. In distinction to previously-shown architectures, although, it doesn’t simply return a prediction. It additionally studies again the RNN’s last state.
decoder_module <- nn_module(
initialize = perform(kind, input_size, hidden_size, num_layers = 1) {
self$kind <- kind
self$rnn <- if (self$kind == "gru") {
nn_gru(
input_size = input_size,
hidden_size = hidden_size,
num_layers = num_layers,
batch_first = TRUE
)
} else {
nn_lstm(
input_size = input_size,
hidden_size = hidden_size,
num_layers = num_layers,
batch_first = TRUE
)
}
self$linear <- nn_linear(hidden_size, 1)
},
ahead = perform(x, state) {
# enter to ahead:
# x is (batch_size, 1, 1)
# state is (1, batch_size, hidden_size)
x <- self$rnn(x, state)
# break up RNN return values
# output is (batch_size, 1, hidden_size)
# next_hidden is
c(output, next_hidden) %<-% x
output <- output$squeeze(2)
output <- self$linear(output)
record(output, next_hidden)
}
)seq2seq module
seq2seq is the place the motion occurs. The plan is to encode as soon as, then name the decoder in a loop.
In the event you look again to decoder ahead(), you see that it takes two arguments: x and state.
Relying on the context, x corresponds to considered one of three issues: last enter, previous prediction, or prior floor reality.
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The very first time the decoder is known as on an enter sequence,
xmaps to the ultimate enter worth. That is totally different from a activity like machine translation, the place you’d go in a begin token. With time collection, although, we’d wish to proceed the place the precise measurements cease. -
In additional calls, we wish the decoder to proceed from its most up-to-date prediction. It is just logical, thus, to go again the previous forecast.
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That mentioned, in NLP a way referred to as “instructor forcing” is often used to hurry up coaching. With instructor forcing, as a substitute of the forecast we go the precise floor reality, the factor the decoder ought to have predicted. We do this solely in a configurable fraction of circumstances, and – naturally – solely whereas coaching. The rationale behind this system is that with out this type of re-calibration, consecutive prediction errors can rapidly erase any remaining sign.
state, too, is polyvalent. However right here, there are simply two potentialities: encoder state and decoder state.
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The primary time the decoder is known as, it’s “seeded” with the ultimate state from the encoder. Word how that is the one time we make use of the encoding.
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From then on, the decoder’s personal earlier state shall be handed. Keep in mind the way it returns two values, forecast and state?
seq2seq_module <- nn_module(
initialize = perform(kind, input_size, hidden_size, n_forecast, num_layers = 1, encoder_dropout = 0) {
self$encoder <- encoder_module(kind = kind, input_size = input_size,
hidden_size = hidden_size, num_layers, encoder_dropout)
self$decoder <- decoder_module(kind = kind, input_size = input_size,
hidden_size = hidden_size, num_layers)
self$n_forecast <- n_forecast
},
ahead = perform(x, y, teacher_forcing_ratio) {
# put together empty output
outputs <- torch_zeros(dim(x)[1], self$n_forecast)$to(gadget = gadget)
# encode present enter sequence
hidden <- self$encoder(x)
# prime decoder with last enter worth and hidden state from the encoder
out <- self$decoder(x[ , n_timesteps, , drop = FALSE], hidden)
# decompose into predictions and decoder state
# pred is (batch_size, 1)
# state is (1, batch_size, hidden_size)
c(pred, state) %<-% out
# retailer first prediction
outputs[ , 1] <- pred$squeeze(2)
# iterate to generate remaining forecasts
for (t in 2:self$n_forecast) {
# name decoder on both floor reality or earlier prediction, plus earlier decoder state
teacher_forcing <- runif(1) < teacher_forcing_ratio
enter <- if (teacher_forcing == TRUE) y[ , t - 1, drop = FALSE] else pred
enter <- enter$unsqueeze(3)
out <- self$decoder(enter, state)
# once more, decompose decoder return values
c(pred, state) %<-% out
# and retailer present prediction
outputs[ , t] <- pred$squeeze(2)
}
outputs
}
)
web <- seq2seq_module("gru", input_size = 1, hidden_size = 32, n_forecast = n_forecast)
# coaching RNNs on the GPU at the moment prints a warning that will muddle
# the console
# see https://github.com/mlverse/torch/points/461
# alternatively, use
# gadget <- "cpu"
gadget <- torch_device(if (cuda_is_available()) "cuda" else "cpu")
web <- web$to(gadget = gadget)The coaching process is primarily unchanged. We do, nonetheless, must resolve about teacher_forcing_ratio, the proportion of enter sequences we need to carry out re-calibration on. In valid_batch(), this could at all times be 0, whereas in train_batch(), it’s as much as us (or slightly, experimentation). Right here, we set it to 0.3.
optimizer <- optim_adam(web$parameters, lr = 0.001)
num_epochs <- 50
train_batch <- perform(b, teacher_forcing_ratio) {
optimizer$zero_grad()
output <- web(b$x$to(gadget = gadget), b$y$to(gadget = gadget), teacher_forcing_ratio)
goal <- b$y$to(gadget = gadget)
loss <- nnf_mse_loss(output, goal)
loss$backward()
optimizer$step()
loss$merchandise()
}
valid_batch <- perform(b, teacher_forcing_ratio = 0) {
output <- web(b$x$to(gadget = gadget), b$y$to(gadget = gadget), teacher_forcing_ratio)
goal <- b$y$to(gadget = gadget)
loss <- nnf_mse_loss(output, goal)
loss$merchandise()
}
for (epoch in 1:num_epochs) {
web$practice()
train_loss <- c()
coro::loop(for (b in train_dl) {
loss <-train_batch(b, teacher_forcing_ratio = 0.3)
train_loss <- c(train_loss, loss)
})
cat(sprintf("nEpoch %d, coaching: loss: %3.5f n", epoch, imply(train_loss)))
web$eval()
valid_loss <- c()
coro::loop(for (b in valid_dl) {
loss <- valid_batch(b)
valid_loss <- c(valid_loss, loss)
})
cat(sprintf("nEpoch %d, validation: loss: %3.5f n", epoch, imply(valid_loss)))
}Epoch 1, coaching: loss: 0.37961
Epoch 1, validation: loss: 1.10699
Epoch 2, coaching: loss: 0.19355
Epoch 2, validation: loss: 1.26462
# ...
# ...
Epoch 49, coaching: loss: 0.03233
Epoch 49, validation: loss: 0.62286
Epoch 50, coaching: loss: 0.03091
Epoch 50, validation: loss: 0.54457It’s attention-grabbing to match performances for various settings of teacher_forcing_ratio. With a setting of 0.5, coaching loss decreases much more slowly; the alternative is seen with a setting of 0. Validation loss, nonetheless, is just not affected considerably.
The code to examine test-set forecasts is unchanged.
web$eval()
test_preds <- vector(mode = "record", size = size(test_dl))
i <- 1
coro::loop(for (b in test_dl) {
output <- web(b$x$to(gadget = gadget), b$y$to(gadget = gadget), teacher_forcing_ratio = 0)
preds <- as.numeric(output)
test_preds[[i]] <- preds
i <<- i + 1
})
vic_elec_jan_2014 <- vic_elec %>%
filter(12 months(Date) == 2014, month(Date) == 1)
test_pred1 <- test_preds[[1]]
test_pred1 <- c(rep(NA, n_timesteps), test_pred1, rep(NA, nrow(vic_elec_jan_2014) - n_timesteps - n_forecast))
test_pred2 <- test_preds[[408]]
test_pred2 <- c(rep(NA, n_timesteps + 407), test_pred2, rep(NA, nrow(vic_elec_jan_2014) - 407 - n_timesteps - n_forecast))
test_pred3 <- test_preds[[817]]
test_pred3 <- c(rep(NA, nrow(vic_elec_jan_2014) - n_forecast), test_pred3)
preds_ts <- vic_elec_jan_2014 %>%
choose(Demand) %>%
add_column(
mlp_ex_1 = test_pred1 * train_sd + train_mean,
mlp_ex_2 = test_pred2 * train_sd + train_mean,
mlp_ex_3 = test_pred3 * train_sd + train_mean) %>%
pivot_longer(-Time) %>%
update_tsibble(key = identify)
preds_ts %>%
autoplot() +
scale_colour_manual(values = c("#08c5d1", "#00353f", "#ffbf66", "#d46f4d")) +
theme_minimal()
Determine 1: One-week-ahead predictions for January, 2014.
Evaluating this to the forecast obtained from final time’s RNN-MLP combo, we don’t see a lot of a distinction. Is that this stunning? To me it’s. If requested to invest in regards to the purpose, I might most likely say this: In all the architectures we’ve used thus far, the primary provider of knowledge has been the ultimate hidden state of the RNN (one and solely RNN within the two earlier setups, encoder RNN on this one). It will likely be attention-grabbing to see what occurs within the final a part of this collection, after we increase the encoder-decoder structure by consideration.
Thanks for studying!
Photograph by Suzuha Kozuki on Unsplash
