Check target boxes input on generalized_rcnn.py

[mmcenta]

Solves #2192 with minimal changes, but I can add checks to the labels and scores fields too. I would also like your opinion on creating a function to make these checks because they can get quite verbose (especially if we check the other fields as well).

Additionally, if we want full validation of the model-specific fields (e.g. masks for Mask R-CNN) we need to specify a check function for each child, perhaps by overriding a _check method or passing it to the __init__ method.

[fmassa]

Thanks a lot for the PR!

I've left a comment, let me know what you think.

About making a function, I would be ok with it, but that can be left for a follow-up PR. You'll also need to add type annotations to it, so that torchscript knows how to handle it.

Also, could you add some tests? Maybe in test/test_models.py or a different test file there, like test/test_models_detection_utlls.py.

[mmcenta]

I fixed the issue and added tests in test/models.py. I chose not to create a new file just yet.
If we choose to expand validation, I can work on the functions and then create separate tests for them.

[fmassa]

Looks great, thanks!