fixed accessing model.val_dataloader to model.trainer.val_dataloader

[danielgafni]

Description

fixing #757

This PR ...

Checklist

• Linked issues (if existing)
• Amended changelog for large changes (and added myself there as contributor)
• Added/modified tests
• Used pre-commit hooks when committing to ensure that code is compliant with hooks. Install hooks with pre-commit install.
  To run hooks independent of commit, execute pre-commit run --all-files

Make sure to have fun coding!

[danielgafni]

Hmm, looks like accessing dataloaders from configure_optimizers can't be done until this pytorch-lightning issue is resolved.

We can either change this logic or wait for pytorch-lightning to fix this.

[jdb78]

Looks like the team is actively working on it.

[jdb78]

We should be able to fix it by removing the trainer dependency and making the lr scheduler optional, i.e. strict=False

[codecov-commenter]

Codecov Report

Merging #758 (4bc9770) into master (910c6d8) will increase coverage by 0.24%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master     #758      +/-   ##
==========================================
+ Coverage   89.76%   90.01%   +0.24%     
==========================================
  Files          24       24              
  Lines        3734     3734              
==========================================
+ Hits         3352     3361       +9     
+ Misses        382      373       -9     

Flag	Coverage Δ
cpu	90.01% <60.00%> (+0.24%)	⬆️
pytest	90.01% <60.00%> (+0.24%)	⬆️

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Impacted Files	Coverage Δ
pytorch_forecasting/models/base_model.py	90.07% <60.00%> (+0.28%)	⬆️
pytorch_forecasting/metrics.py	93.45% <0.00%> (+0.23%)	⬆️
pytorch_forecasting/utils.py	81.94% <0.00%> (+1.38%)	⬆️
pytorch_forecasting/models/nn/rnn.py	92.40% <0.00%> (+5.06%)	⬆️

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