Re-adding load_submodules

[ericspod]

Description

This restores load_modules for now to resolve importation issues. Doing a two-pass loading process seems to allow some references which normally Python wouldn't permit. The fact that these issues weren't caught in testing is rather strange, and only showed up when symbols in monai.apps were references in bundles. I'm not sure if this is all related to circular imports, if parts of MONAI aren't being tested properly, or the way the symbol resolution works within monai.bundle should be improved.

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.

[KumoLiu]

Let's also include this test back then let me trigger more tests.
https://github.com/Project-MONAI/MONAI/blob/1.4.0rc9/tests/min_tests.py#L235

[ericspod]

Let's also include this test back then let me trigger more tests.
https://github.com/Project-MONAI/MONAI/blob/1.4.0rc9/tests/min_tests.py#L235

Ok that's back in now.

[KumoLiu]

/build