Add divergence metrics

[kzkadc]

Description: added KL and JS divergences between two categorical predictions

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 a lot adding new metrics @kzkadc !

[vfdev-5]

@kzkadc can you please check this failure with pytorch 1.5.1: https://github.com/pytorch/ignite/actions/runs/8730344048/job/23953994151

TypeError: kl_div() got an unexpected keyword argument 'log_target'

Looks like log_target arg was not added in 1.5.1, can you see what would be the effort to support 1.5.1.
We have two options: a) fix ignite code if this is a minimal fix, b) drop support for 1.5.1 in our pytorch versions tests and replace it with a minimal later version. Option a) would be preferable but the fix should not be a large update of the code.

[kzkadc]

Thanks. The kl_div's behaviour in 1.5.1 is equivalent to log_target=False (default) in the current version.
We can fix it by passing the target variable to kl_div as a probabilities.

[vfdev-5]

we can also make a small if-else depending on pytorch version. For example:

ignite/ignite/metrics/gan/fid.py

Line 15 in f431e60

if Version(torch.__version__) <= Version("1.7.0"):

Using log_target may be has better computation result stability (to confirm), so, for recent pytorch it would be better to use this option if it makes more sense to use vs keeping default log_target=False