Finish TODOs in NHiTs and NBEATs

[gdevos010]

Fixes #.
#954

Summary

Made NHiTs activation configurable between relu and gelu
Apply NHiTs dropout and batch_norm to NBEATs

[codecov-commenter]

Codecov Report

Merging #955 (3cc47f1) into master (adb66fd) will increase coverage by 0.00%.
The diff coverage is 93.10%.

❗ Current head 3cc47f1 differs from pull request most recent head e1ed821. Consider uploading reports for the commit e1ed821 to get more accurate results

@@           Coverage Diff           @@
##           master     #955   +/-   ##
=======================================
  Coverage   92.61%   92.61%           
=======================================
  Files          74       74           
  Lines        7404     7423   +19     
=======================================
+ Hits         6857     6875   +18     
- Misses        547      548    +1     

Impacted Files	Coverage Δ
darts/models/forecasting/nbeats.py	98.05% <90.47%> (-1.22%)	⬇️
darts/models/forecasting/nhits.py	99.24% <100.00%> (+0.82%)	⬆️
darts/timeseries.py	88.00% <0.00%> (-0.06%)	⬇️
...arts/models/forecasting/torch_forecasting_model.py	89.30% <0.00%> (-0.05%)	⬇️
darts/models/forecasting/forecasting_model.py	96.78% <0.00%> (+0.01%)	⬆️

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[hrzn]

This looks great, thanks a lot! I have a couple of minor comments. One of them would be to support a couple more activation functions (the ones in PyTorch that can be easily added). Also how about making the activation configurable in N-BEATS as well, while we're at it? :)

Finally, please add a line to the CHANGELOG.md file listing the changes. You're most welcome to put your name in there too if you want.

[hrzn]

Thanks, new changes look good. I agree it's probably better to leave batch norm out if it seems to perform badly. Could you maybe add/modify a couple of test cases, to integrate a couple of the new parameters?

[gdevos010]

@hrzn I added an activation test. let me know if I should add any others

[gdevos010]

@hrzn Im going to need some help with the failing test. I don't get any errors on my machine.
FAILED darts/tests/models/filtering/test_filters.py::GaussianProcessFilterTestCase::test_gaussian_process

[hrzn]

Good one, thanks! Will merge.