Add get_param method to ParamGroupScheduler

[sadra-barikbin]

In PR #2704 , in order to be able to have multiple groups in FastaiLRFinder when step_mode != 'exp', I suggested this code:
https://github.com/pytorch/ignite/pull/2704/files#r975195661

which uses ParamGroupScheduler. FastaiLRFinder calls get_param on its scheduler in

ignite/ignite/handlers/lr_finder.py

Line 160 in daa3c96

lr = self._lr_schedule.get_param() # type: ignore[union-attr]

but ParamGroupScheduler has not this method. This PR adds that method.

[vfdev-5]

Thanks for the PR @sadra-barikbin
looks good to me.
Left a small comment on updating docstring

[vfdev-5]

I close and reopen the PR to restart the CI

[sadra-barikbin]

@vfdev-5 , linkcheck job failed due to https://magellium.com link in docs. What is it?

[vfdev-5]

@sadra-barikbin it is from about.rst page: https://github.com/pytorch/ignite/blob/f9cd677de7aa87e9ecb9ad658c7170dddb85f444/docs/source/about.rst
we can ignore it for now if the failure remains we can remove the link