harmonization and clarification of dice losses variants docs and associated tests

[Lucas-rbnt]

Description

This PR aims to clarify and harmonise the code for the DiceLoss variants in the monai/losses/dice.py file. With the to_onehot_y softmax and sigmoid arguments, I didn't necessarily understand the ValueError that occurred when I passed a target of size NH[WD]. I had a bit of trouble reading the documentation and understanding it. I thought that they had to be the same shape as they are displayed, unlike the number of dimensions in the input, so I added that.
Besides, in the documentation is written:

"""
raises:
      ValueError: When number of channels for target is neither 1 nor the same as input.

"""

Trying to reproduce this, we give an input with a number of channels $N$ and target a number of channels of $M$, with $M \neq N$ and $M > 1$.

loss = DiceCELoss()
input = torch.rand(1, 4, 3, 3)
target = torch.randn(1, 2, 3, 3)
loss(input, target)
>: AssertionError: ground truth has different shape (torch.Size([1, 2, 3, 3])) from input (torch.Size([1, 4, 3, 3]))

This error in the Dice is an AssertionError and not a ValueError as expected and the explanation can be confusing and doesn't give a clear idea of the error here. The classes concerned and harmonised are DiceFocalLoss, DiceCELoss and GeneralizedDiceFocalLoss with the addition of tests that behave correctly and handle this harmonisation.

Also, feel free to modify or make suggestions regarding the changes made in the docstring to make them more understandable (in my opinion, but other readers and users will probably have a different view).

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.

[KumoLiu]

Thank you for this pull request, overall, the updates are looking good to me. However, I do have two minor comments inline for your consideration.

[KumoLiu]

/build