make tensorflow dependency optional

[jeongyoonlee]

Proposed changes

tensorflow has strict dependency requirements including numpy~=0.19, which results in issues e.g. #338. Currently, DragonNet is the only model that requires tensorflow. Therefore, it'd be better to make it optional so that users who do not need DragonNet can avoid tensorflow related issues.

This PR includes changes as follows:

• Make tensorflow dependency optional by moving DragonNet out from inference.nn and to a new inference.tf module.
• Update DragonNet with new TF2.0 API
• Update envs/environment-py3x.yml without tensorflow related dependencies
• Add envs/environment-tf-py3x.yml with tensorflow related dependencies
• Add envs/environment-[tf]-py39.yml for Python 3.9
• Update README.md accordingly

DragonNet will be available with pip install causalml[tf], which will install tensorflow.

Types of changes

What types of changes does your code introduce to CausalML?
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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I have read the CONTRIBUTING doc
• I have signed the CLA
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Further comments

N/A

[paullo0106]

Looks good! I have got a question

[alee103]

Hello Jeong! I am new to Python and I am running into this issue currently.

I was wondering whether the proposed changes you mentioned in your initial comment have been implemented yet? Would you have advice on how to circumvent this error (e.g., reinstall the packages)?