Fix instantiate for object instantiation with attribute path

[surajpaib]

Fixes #5865

-- also fixes #5868 to unblock premerge

Description

A few sentences describing the changes proposed in this pull request.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.

[wyli]

Thanks, could you please signoff the commit with "git commit --signoff" https://github.com/Project-MONAI/MONAI/blob/dev/CONTRIBUTING.md#signing-your-work

The flake8 style error is irrelevant, I'll change the style config when merging this PR

[wyli]

/build