Fix get_backend bug and add clear_device_cache function

[NurmaU]

What does this PR do?

This PR introduces 2 updates:

1. adds new clear_device_cache function in memory.py to replace repetitive code. This also adds check for mps devices in function find_executable_batch_size. This feature was tested with tests/test_memory_utils.py, which had bug.
2. fixes bug in get_backend function where function's 3rd outputs must be callable object. This bug caused failures in tests/test_memory_utils.py on MacOS.

Before submitting

• [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• [ ] Was this discussed/approved via a Github issue or the forum? Please add a link to it if that's the case.
• [ ] Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and here are tips on formatting docstrings.
• [ ] Did you write any new necessary tests?

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