fix that cannot correctly set bn layer channels

[wurining]

When setting the hidden_channels as a integer, function add_bn_layer was passed a error parameter which caused TypeError as below

Traceback (most recent call last):
  File "/***/sensorium_2023/main.py", line 143, in <module>
    full_3d_model = make_video_model(
  File "/***/sensorium_2023/sensorium_2023/sensorium/models/make_model.py", line 141, in make_video_model
    core = Basic3dCore(**core_dict)
  File "/***/conda_envs/envs/sensorium/lib/python3.9/site-packages/neuralpredictors/layers/cores/conv3d.py", line 169, in __init__
    self.add_bn_layer(layer=layer, hidden_channels=hidden_channels[0])
TypeError: 'int' object is not subscriptable

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 7.38%. Comparing base (a64a9cc) to head (d59eaab).

Files	Patch %	Lines
neuralpredictors/layers/cores/conv3d.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #210   +/-   ##
=====================================
  Coverage   7.38%   7.38%           
=====================================
  Files         58      58           
  Lines       5947    5947           
  Branches    1006    1006           
=====================================
  Hits         439     439           
  Misses      5478    5478           
  Partials      30      30           

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[MaxFBurg]

Thank you very much for the example and fix @wurining - we highly appreciate your contribution!