[Bugfix] [Relay] fix a bug of printing dataflow pattern

[kfeng123]

When recursion of dataflow patterns is used, the pattern graph may not be a DAG. If recursion is encountered, ReprPrint of dataflow pattern may fall in a dead loop.
This PR solves the bug of dataflow pattern printing, and is the first PR for the pre-RFC.

[tvm-bot]

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• cc @shingjan _{See #10317 for details}

_{Generated by tvm-bot}

[kfeng123]

cc @masahi @mbrookhart

[masahi]

btw, does the dataflow pattern support recursion (loop) at all?

[masahi]

Please document what you mean by "recursed_patterns". Probably "recursive_patterns" would be more correct.

[kfeng123]

Documentation added. To be consistent with the printed text, I choose to name this variable to be auxiliary_patterns

[masahi]

Remove "is" and "are" above

[kfeng123]

Done

[kfeng123]

btw, does the dataflow pattern support recursion (loop) at all?

Surprisingly, the answer is yes. Indeed, I was thinking to add the support for recursion, only to find that the existing dataflow pattern language has alreadly (silently) support it.
But currently recursion may only implemented in C++, not python.
The following is an example taken from the pre-RFC doc:

TVM_REGISTER_GLOBAL("relay.dataflow_pattern.my_pattern")
    .set_body_typed([]() {
    DFPattern dense_pattern = IsOp("nn.dense")({IsWildcard(), IsWildcard()});
    ObjectPtr<CallPatternNode> the_pattern_ptr = make_object<CallPatternNode>();
    the_pattern_ptr->op = IsOp("cast");
    the_pattern_ptr->args.clear();
    CallPattern the_pattern = CallPattern(the_pattern_ptr);
    AltPattern or_pattern{the_pattern, dense_pattern};
    the_pattern_ptr->args.push_back(or_pattern);
    //LOG(INFO) << PrettyPrint(the_pattern); // TODO: BUG!
  return the_pattern;
});

This simple pattern matches a nn.dense followed by an arbitrary number of cast. You can test this pattern via the following python code:

class TheRewrite(DFPatternCallback):
    def __init__(self):
        super(TheRewrite, self).__init__(rewrite_once = True)
        pattern = tvm.get_global_func("relay.dataflow_pattern.my_pattern")()
        self.pattern = pattern
    def callback(self, pre, post, node_map):
        return relay.nn.relu(post)

mod = create_model() # define a model
the_rewrite = TheRewrite()
out = rewrite(the_rewrite, mod["main"])

Another application of recursion is PR #15362, which I do not know how to achieve without recursion. That PR is useful, and can really improve the computational graph for some quantized models.

I would like to examine the pattern matching code further in the following days.
If I find any bug with pattern recursion, I would like to raise PRs.

[kfeng123]

@tvm-bot rerun