Add Entropy metric

[kzkadc]

Description: adds Entropy metric.
Entropy is often used when evaluating uncertainty of classification predictions.
It is computed as $H=\sum_c - p_c \log p_c$, where $p_c$ is the predicted probability for $c$-th class.

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]

For distributed config tests, could you please rewrite them using new testing formalism that we are trying to adopt. Here is an example of the code to inspire of:

ignite/tests/ignite/metrics/test_recall.py

Lines 422 to 426 in 5fe7443

	@pytest.mark.usefixtures("distributed")
	class TestDistributed:
	@pytest.mark.parametrize("average", [False, "macro", "weighted", "micro"])
	@pytest.mark.parametrize("n_epochs", [1, 2])
	def test_integration_multiclass(self, average, n_epochs):

Here is a PR showing how to pass from old code to the new one:
https://github.com/pytorch/ignite/pull/3208/files#diff-c56c264ef288f88e5738e9ad22de66dffd4c58d2e656eb62e8dbaa678672317d

Thanks!

[vfdev-5]

@kzkadc thanks for the PR, lgtm now

Updating distributed config test to the new code can be done in a follow-up PR if you would like.
Let's see if CI is passing and merge this PR

[vfdev-5]

@kzkadc can you please run bash tests/run_code_style.sh fmt to fix all code formatting issues, thanks!
https://github.com/pytorch/ignite/actions/runs/8390870373/job/22979934843?pr=3210

[vfdev-5]

@kzkadc this failure is related: https://github.com/pytorch/ignite/actions/runs/8391979913/job/22983553399?pr=3210#step:9:345

         y_preds = idist.all_gather(y_preds)
        y_true = idist.all_gather(y_true)
    
        assert "entropy" in engine.state.metrics
        res = engine.state.metrics["entropy"]
    
>       true_res = np_entropy(y_preds.numpy())
E       TypeError: can't convert xla:0 device type tensor to numpy. Use Tensor.cpu() to copy the tensor to host memory first.

tests/ignite/metrics/test_entropy.py:102: TypeError