Fix issue 650

[vincewu51]

Proposed changes

Fix issue by add back the version in init.py

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[jeongyoonlee]

As I left a comment in #650, let's update the notebooks to use importlib.metadata.version('causalml') instead of causalml.__version__.

[vincewu51]

Sounds good. Fixed few other notebook too.

[ras44]

@vincewu51 I duplicated some work here, please ignore the commits linked to above in ras44/causalml that reference this issue

[jeongyoonlee]

LGTM. Thanks!