[Logging] Switch from Standard Logging to Loguru (#1521)

## Purpose ##
As loguru is used in some places, we update other logging instances for
consistency with its behavior

## Changes ##
* metrics/logger.py: Change `level: Optional[int]` to `level:
Optional[Union[int, str]]` to better support loguru's logging level
behavior
* Update `PythonLogger._create_default_logger` to use loguru.logger
* Simplify `SparsificationGroupLogger` by replacing the `python` union
type with `bool`, aligning it with `log_python` in `LoggerManager` and
removing ambiguity

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Co-authored-by: Dipika Sikka <dipikasikka1@gmail.com>
Co-authored-by: shanjiaz <43143795+shanjiaz@users.noreply.github.com>

Author: 3 people