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Transfer learning refers to the process of pre-training a flexible model on a large dataset and using it later on other data with little to no training. It is one of the most outstanding 🚀 achievements in Machine Learning 🧠 and has many practical applications. For time series forecasting, the technique allows you to get lightning-fast predictions ⚡ bypassing the tradeoff between accuracy and speed (more than 30 times faster than our alreadsy fast autoARIMA for a similar accuracy). This notebook shows how to generate a pre-trained model and store it in a checkpoint to make it available to forecast new time series never seen by the model. Table of Contents
1. Installing NeuralForecast/DatasetsForecast
2. Load M4 Data
3. Instantiate NeuralForecast core, Fit, and save
4. Load pre-trained model and predict on AirPassengers
5. Evaluate Results
 You can run these experiments using GPU with Google Colab. Open In Colab

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1. Installing Libraries

!pip install datasetsforecast neuralforecast

import logging

import numpy as np
import pandas as pd
import torch
from datasetsforecast.m4 import M4
from neuralforecast.core import NeuralForecast
from neuralforecast.models import NHITS
from neuralforecast.utils import AirPassengersDF
from utilsforecast.losses import mae
from utilsforecast.plotting import plot_series

logging.getLogger("pytorch_lightning").setLevel(logging.WARNING)
This example will automatically run on GPUs if available. Make sure cuda is available. (If you need help to put this into production send us an email or join or community, we also offer a fully hosted solution) 
torch.cuda.is_available()

True

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2. Load M4 Data

The M4 class will automatically download the complete M4 dataset and process it. It return three Dataframes: Y_df contains the values for the target variables, X_df contains exogenous calendar features and S_df contains static features for each time-series (none for M4). For this example we will only use Y_df. If you want to use your own data just replace Y_df. Be sure to use a long format and have a simmilar structure than our data set. 
Y_df, _, _ = M4.load(directory='./', group='Monthly', cache=True)
Y_df['ds'] = pd.to_datetime(Y_df['ds'])
Y_df
unique_iddsy
0M11970-01-01 00:00:00.0000000018000.0
1M11970-01-01 00:00:00.0000000028350.0
2M11970-01-01 00:00:00.0000000038570.0
3M11970-01-01 00:00:00.0000000047700.0
4M11970-01-01 00:00:00.0000000057080.0
…………
11246406M99991970-01-01 00:00:00.0000000834200.0
11246407M99991970-01-01 00:00:00.0000000844300.0
11246408M99991970-01-01 00:00:00.0000000853800.0
11246409M99991970-01-01 00:00:00.0000000864400.0
11246410M99991970-01-01 00:00:00.0000000874300.0

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3. Model Train and Save

Using the NeuralForecast.fit method you can train a set of models to your dataset. You just have to define the input_size and horizon of your model. The input_size is the number of historic observations (lags) that the model will use to learn to predict h steps in the future. Also, you can modify the hyperparameters of the model to get a better accuracy. 
horizon = 12
stacks = 3
models = [NHITS(input_size=5 * horizon,
                h=horizon,
                max_steps=100,
                stack_types = stacks*['identity'],
                n_blocks = stacks*[1],
                mlp_units = [[256,256] for _ in range(stacks)],
                n_pool_kernel_size = stacks*[1],
                batch_size = 32,
                scaler_type='standard',
                n_freq_downsample=[12,4,1],
                enable_progress_bar=False,
                interpolation_mode="nearest",
               )]
nf = NeuralForecast(models=models, freq='ME')
nf.fit(df=Y_df)

INFO:lightning_fabric.utilities.seed:Seed set to 1
Save model with core.NeuralForecast.save method. This method uses PytorchLightning save_checkpoint function. We set save_dataset=False to only save the model. 
nf.save(path='./results/transfer/', model_index=None, overwrite=True, save_dataset=False)

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4. Transfer M4 to AirPassengers

We load the stored model with the core.NeuralForecast.load method, and forecast AirPassenger with the core.NeuralForecast.predict function. 
fcst2 = NeuralForecast.load(path='./results/transfer/')

c:\Nixtla\Repositories\neuralforecast\neuralforecast\common\_base_model.py:133: UserWarning: NHITS is a univariate model. Parameter n_series is ignored.
  warnings.warn(
INFO:lightning_fabric.utilities.seed:Seed set to 1

# We define the train df. 
Y_df = AirPassengersDF.copy()
mean = Y_df[Y_df.ds<='1959-12-31']['y'].mean()
std = Y_df[Y_df.ds<='1959-12-31']['y'].std()

Y_train_df = Y_df[Y_df.ds<='1959-12-31'] # 132 train
Y_test_df = Y_df[Y_df.ds>'1959-12-31']   # 12 test

Y_hat_df = fcst2.predict(df=Y_train_df)
Y_hat_df.head()
unique_iddsNHITS
01.01960-01-31422.038757
11.01960-02-29424.678040
21.01960-03-31439.538879
31.01960-04-30447.967072
41.01960-05-31470.603333
plot_series(Y_train_df, Y_hat_df)

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5. Evaluate Results

We evaluate the forecasts of the pre-trained model with the Mean Absolute Error (mae). MAE=1Horizon∑τ∣yτ−y^τ∣ \qquad MAE = \frac{1}{Horizon} \sum_{\tau} |y_{\tau} - \hat{y}_{\tau}|\qquad 
fcst_mae = mae(Y_test_df.merge(Y_hat_df), models=['NHITS'])['NHITS'].item()
print(f'NHITS     MAE: {fcst_mae:.3f}')
print('ETS       MAE: 16.222')
print('AutoARIMA MAE: 18.551')

NHITS     MAE: 17.245
ETS       MAE: 16.222
AutoARIMA MAE: 18.551