Fix mypy typing errors in strategies/fully_sharded.py

[Jungwon-Lee]

What does this PR do?

Part of #13445.

Fix typing errors in pytorch_lightning/strategies/fully_sharded.py in issue #13445.

Does your PR introduce any breaking changes? If yes, please list them.

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

PR review

Anyone in the community is welcome to review the PR.
Before you start reviewing, make sure you have read the review guidelines. In short, see the following bullet-list:

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

Did you have fun?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

Merging #13941 (f308a1f) into master (e513638) will decrease coverage by 3%.
The diff coverage is 47%.

❗ Current head f308a1f differs from pull request most recent head 418eb82. Consider uploading reports for the commit 418eb82 to get more accurate results

@@            Coverage Diff             @@
##           master   #13941      +/-   ##
==========================================
- Coverage      80%      76%      -3%     
==========================================
  Files         103      335     +232     
  Lines        6833    26379   +19546     
==========================================
+ Hits         5446    20136   +14690     
- Misses       1387     6243    +4856