Fix docs build

[DevPranjal]

Fixes #2464

Description: After #2457, the docs builds failed due to mention of the new file (defaults.rst) in the master conf.py which is used to build docs for all other versions as well. For now, this PR skips the copy operation when the file is not found to make the conf.py compatible with other versions.

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]

@DevPranjal thanks for the fix ! Have you tried to build docs locally using sphinxversionning to ensure that everything is working ?

[DevPranjal]

Yes, the docs were built for other versions as well, and the html files were running well!
There were some warnings that do not seem related to this PR:

/tmp/tmp9gd7uf6dsphinxcontrib_versioning/61b425286ba89092ba0390b9c2d5a26d3d579ef2/ignite/engine/engine.py:docstring of ignite.engine.engine.Engine:1: WARNING: duplicate object description of ignite.engine.engine.Engine, other instance in engine, use :noindex: for one of them

/tmp/tmp9gd7uf6dsphinxcontrib_versioning/5837b2d40ba8302e986a5a5115b734990cbe201b/ignite/handlers/checkpoint.py:docstring of ignite.handlers.checkpoint.ModelCheckpoint:55: WARNING: Block quote ends without a blank line; unexpected unindent.

/tmp/tmp9gd7uf6dsphinxcontrib_versioning/84b5d413b68cf0407cb7bd027bec194809a5db2c/docs/source/engine.rst:2: WARNING: Duplicate explicit target name: "pytorch documentation".

[vfdev-5]

LGTM, thanks a lot @DevPranjal !