[Unity] Minor updates to DataFlowBlockRewrite

[masahi]

• relax/binding_rewrite.py is not imported by default, so we get errors like AttributeError: <class 'tvm.runtime.object.Object'> has no attribute add if we try to pass the C++ object to python

• DataflowBlock member variable shouldn't be optional, but it is declared with Optional<DataflowBlock> dfb_ and accessed via dfb_.value() all over the places.

• root_fn_ should be optional (e.g. when DataFlowBlockRewrite is used inside another ExprMutator) , but it's always updated via root_fn_.value() without checking

• Other minor style updates that hopefully improve readability.

@ganler @tqchen

[tvm-bot]

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• cc @quic-sanirudh _{See #10317 for details}

_{Generated by tvm-bot}

[masahi]

I just realized that I didn't need to use DataflowBlockRewrite for my purpose. But keeping this PR for now.