Remove manual `torch.no_grad` and train eval mode in LightningModule

[aniketmaurya]

PL LightningModule automatically sets train and eval mode and torch.no_grad during the training and validation steps.

[codecov[bot]]

Codecov Report

Merging #1100 (13bb3e7) into master (743cb7d) will increase coverage by 0.00%. The diff coverage is 80.00%.

@@           Coverage Diff           @@
##           master    #1100   +/-   ##
=======================================
  Coverage   85.43%   85.44%           
=======================================
  Files         301      301           
  Lines        6600     6594    -6     
=======================================
- Hits         5639     5634    -5     
+ Misses        961      960    -1     

Flag	Coverage Δ
unittests	85.44% <80.00%> (+<0.01%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...odels/ross/efficientdet/lightning/model_adapter.py	37.93% <0.00%> (+1.26%)	:arrow_up:
icevision/models/mmdet/lightning/model_adapter.py	96.96% <100.00%> (-0.26%)	:arrow_down:
...sion/models/torchvision/lightning_model_adapter.py	100.00% <100.00%> (ø)
...dels/ultralytics/yolov5/lightning/model_adapter.py	100.00% <100.00%> (ø)

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[potipot]

Any idea why black formatting check might failing here?