Fix logging issues that can arise when importing flaml

[kernelmethod]

Previously, FLAML would automatically set its own loglevel when imported, overriding any other logging settings that may previously have been set by users of the library. FLAML will now check whether a loglevel has been explicitly set before setting its own.

In addition, the flaml.automl warning is now emitted using warnings rather than logging.warning. This falls more in line with other FLAML submodules (e.g. flaml.ml).

Taken in conjunction, these two changes make it a lot easier for downstream users to suppress warnings emitted by FLAML upon importing the library, either by configuring logging prior to importing flaml or by using the somewhat more idiomatic

with warnings.catch_warnings(action="ignore"):
    import flaml

This change also helps to ensure that FLAML's defaults won't override users' explicit logging settings.

Why are these changes needed?

These changes make it easier for downstream users to configure FLAML's logging utilities as they see fit, and make it easier to suppress certain warnings that may occur when importing flaml for the first time.

Related issue number

N/A

Checks

• I've used pre-commit to lint the changes in this PR (note the same in integrated in our CI checks).
• I've included any doc changes needed for https://microsoft.github.io/FLAML/. See https://microsoft.github.io/FLAML/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[thinkall]

Thank you for the PR, @kernelmethod ! LGTM!