New loss functions, refactored engines to only implement forward_method, quality fixes

[georgeyiasemis]

Implements:

• New loss functions (NMSE, NRMSE, NMAE, SobelGradL1Loss, SobelGradL2Loss)
• mri_models performs _do_iteration method, child engines perform forward_function which returns output_image and/or output_kspace
• Changes/Additions in mri_transforms
  • Padding computed as a Tensor with ComputePadding transform (this is helpful when cropping image and tranforming to kspace)
  • ApplyPadding transform
  • ComputeImage transform choices of mod output or not
  • RenameKeys transform
  • Normalize split to ComputeScalingFactor and Normalize
• Some quality changes
• Some documentation changes

[codecov]

Codecov Report

Base: 82.02% // Head: 81.91% // Decreases project coverage by -0.11% ⚠️

Coverage data is based on head (fbe7716) compared to base (1a8ddf5).
Patch coverage: 92.57% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #226      +/-   ##
==========================================
- Coverage   82.02%   81.91%   -0.12%     
==========================================
  Files          74       77       +3     
  Lines        5585     5589       +4     
==========================================
- Hits         4581     4578       -3     
- Misses       1004     1011       +7     

Impacted Files	Coverage Δ
direct/nn/cirim/cirim.py	98.27% <ø> (ø)
direct/nn/cirim/cirim_engine.py	95.45% <50.00%> (ø)
direct/exceptions.py	62.50% <85.71%> (+21.32%)	⬆️
direct/nn/mri_models.py	91.30% <85.98%> (-2.31%)	⬇️
direct/data/mri_transforms.py	97.00% <89.47%> (-2.29%)	⬇️
direct/functionals/grad.py	97.95% <97.95%> (ø)
direct/common/subsample.py	84.63% <100.00%> (ø)
direct/data/datasets.py	87.46% <100.00%> (-0.99%)	⬇️
direct/data/datasets_config.py	100.00% <100.00%> (ø)
direct/data/h5_data.py	65.06% <100.00%> (+0.24%)	⬆️
... and 20 more

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