[TOPI] Prevent scheduling the same operation more than once

[masahi]

Split from #1548.

Future updates to NNVM operator fusion may introduce cases where the same op can be reached from more than one route during output-to-input traversal. This PR makes sure an operation is scheduled only once.

This PR will be followed by #1548, where a relevant test case is included.

[tqchen]

Thanks @masahi , this is merged!

[merrymercy]

You should handle this logic in the utility function traverse_inline.
Currently the traverse inline logic takes much redundant code in TOPI. I recommend that developers should switch to use this utility function.

[masahi]

Yeah it was pain to add all those scheduled_ops stuff to all backends.

But it is not a bug, right?

I was not in a mood to refactor all traverse logic to use traverse_inline.

[masahi]

When I looked at your traverse_inline function, I thought I need to pass around scheduled_ops to traverse_inline function. I didn't want to make this change, so I chose a more straightforward approach.

[merrymercy]

Here is my local version. Possibly you can update it for arm_cpu only in your NNVM fusion PR.

def traverse_inline(s, final_op, callback):
    """Traverse computation graph and do auto inline

    Parameters
    ----------
    s: schedule
        The schedule
    final_op: Operation
        The final output operator.
    callback: callable
        The callback function on each op
    """
    visited = set()

    def _traverse(op):
        if op in visited:
            return
        visited.add(op)
        if tag.is_injective(op.tag):
            if op not in s.outputs:
                s[op].compute_inline()
            for tensor in op.input_tensors:
                if tensor.op.input_tensors:
                    traverse_inline(s, tensor.op, callback)
        callback(op)

    _traverse(final_op)

[masahi]

It should be

 for tensor in op.input_tensors:
        if tensor.op.input_tensors:
               _traverse(tensor.op)

no?

[masahi]

I updated #1548 according to your comment.

[merrymercy]

Aha, yes..