Add HarmonicMean metric to ignite.metrics

[leelakrishnaRajasimha]

Fixes #3648

This PR introduces a new metric HarmonicMean to compute the harmonic mean of input values.

Features:

• Device support
• Distributed training support via sync_all_reduce
• Unit tests
• Documentation

[Prathamesh8989]

Hey @leelakrishnaRajasimha ,

Nice addition! One small thing I noticed in harmonic_mean.py within the reset() method: _num_examples is currently initialized as a Python integer (self._num_examples = 0), but it's later included in the @sync_all_reduce("_sum_reciprocal", "_num_examples") decorator used in the compute() method.

Since sync_all_reduce usually operates on tensors for distributed reduction, would it be safer to store _num_examples as a tensor on self._device for consistency with _sum_reciprocal?

For example, this could be adjusted in reset() where _num_examples is initialized.