[TVMScript][Relax] Allow return statement in DataflowBlock

[Lunderberg]

Prior to this commit, TVMScript required the return value of a Relax to be specified outside of any with R.dataflow() blocks. This resulted in a common pattern, where the return value of a function was first called with R.output(ret_value), to mark ret_value as a tvm::relax::Var instead of a tvm::relax::DataflowVar, followed immediately by a return ret_value statement.

This commit updates the TVMScript parser to allow a return statement inside a with R.dataflow() block. This is syntactic sugar that is equivalent to calling R.output, followed by a return.

With this change, the following two TVMScript examples are now equivalent. (Prior to this change, the return_inside_dataflow example would raise an error during parsing.)

@R.function(private=True)
def output_then_return(A: R.Tensor):
    with R.dataflow():
        B = R.add(A, A)
        C = R.multiply(B, B)
        R.output(C)

    return C

@R.function(private=True)
def return_inside_dataflow(A: R.Tensor):
    with R.dataflow():
        B = R.add(A, A)
        C = R.multiply(B, B)
        return C