Cleaned up some tests in test_loss.py

[asmayer]

Addresses #2522

Description:

Cleans up some tests in test_loss.py

Something worth noting:
test_compute_on_criterion and test_compute were identical. I addressed this by having test_compute_on_criterion call test_compute but perhaps this test should be removed entirely

Check list:

• New tests are added (if a new feature is added)
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)

[vfdev-5]

Thanks for the PR @asmayer
Please, run code formatting tools to make sure if the code is properly formatted: https://github.com/pytorch/ignite/blob/master/CONTRIBUTING.md#formatting-code

Concerning your note about test_compute_on_criterion and test_compute were identical.
Actually, it is not completely true.
test_compute is using nll_loss which is functional NLL implementation and it is a function. test_compute_on_criterion is using nn.NLLLoss() which nn.Module.

[asmayer]

Hey @vfdev-5,

Thanks for the review. I committed some changes that address the two issues you pointed out. Both of which could have have been easy avoided by reading the contributing guidelines along with the source code more carefully. I am a bit new to contributing to open source but this was simply careless on my end. I apologize for all of that.

[vfdev-5]

Thanks for the updates @asmayer
I left few more comments to adress

[vfdev-5]

Thanks for the PR @asmayer and sorry for such delay 🙏