Update tests for trainer.fit returning None in PL 1.3.0

[akihironitta]

What does this PR do?

Found in #633.

This PR updates the tests comparing with return value of trainer.fit(...) since trainer.fit always returns None from PL 1.3. Lightning-AI/pytorch-lightning#7237

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[codecov]

Codecov Report

Merging #641 (172a45c) into master (f0950d2) will increase coverage by 0.36%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #641      +/-   ##
==========================================
+ Coverage   71.46%   71.83%   +0.36%     
==========================================
  Files         118      118              
  Lines        7129     7129              
==========================================
+ Hits         5095     5121      +26     
+ Misses       2034     2008      -26     

Flag	Coverage Δ
cpu	71.83% <ø> (+0.36%)	⬆️
pytest	71.83% <ø> (+0.36%)	⬆️

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

Impacted Files	Coverage Δ
pl_bolts/datasets/cifar10_dataset.py	72.04% <0.00%> (-25.81%)	⬇️
pl_bolts/datasets/base_dataset.py	81.81% <0.00%> (-13.64%)	⬇️
pl_bolts/models/rl/dqn_model.py	80.45% <0.00%> (+0.57%)	⬆️
pl_bolts/models/rl/reinforce_model.py	89.47% <0.00%> (+0.75%)	⬆️
pl_bolts/datamodules/experience_source.py	95.90% <0.00%> (+0.81%)	⬆️
...l_bolts/models/rl/vanilla_policy_gradient_model.py	95.90% <0.00%> (+0.81%)	⬆️
pl_bolts/models/rl/per_dqn_model.py	86.88% <0.00%> (+1.63%)	⬆️
pl_bolts/models/rl/noisy_dqn_model.py	82.97% <0.00%> (+2.12%)	⬆️
...l_bolts/models/self_supervised/byol/byol_module.py	51.57% <0.00%> (+28.42%)	⬆️
pl_bolts/callbacks/byol_updates.py	100.00% <0.00%> (+31.81%)	⬆️
... and 1 more

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[akihironitta]

This PR is now ready for your review @Borda

[Borda]

Perfect, thank you!