[Unity][Graph matching] Automatically add used-by constraints for is_op pattern

[masahi]

Compared to the typical, structural matching (e.g. matching is_op based pattern against CallNode), graph pattern matching requires additionally specifying used_by constraints. This leads to an obvious redundancy in the usage:

matmul1 = is_op("relax.matmul")(inp_pat, Q_weight_pat)
matmul2 = is_op("relax.matmul")(inp_pat, K_weight_pat)
matmul3 = is_op("relax.matmul")(inp_pat, V_weight_pat)

inp_pat.used_by(matmul1, 0)
inp_pat.used_by(matmul2, 0)
inp_pat.used_by(matmul3, 0)

Q_weight_pat.only_used_by(matmul1, 1)
K_weight_pat.only_used_by(matmul2, 1)
V_weight_pat.only_used_by(matmul3, 1)

This PR hacks is_op constructor by automatically adding such used_by constraints between caller and callee patterns. See the simplified test case for QOL improvement.

@ganler @sunggg

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

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[masahi]

This gets removed since it is not used anywhere but it touches PatternContext::Current() (which is now Optional)

[ganler]

I understand used_by can be removed now as it is implied by a function call. However, only_used_by is a stronger constraint that cannot be implied via a fn call right?

[ganler]

But yeah I guess you meant oftentimes we don't fold Q, K, and V so it is fine if they are being used by others at the same time.

[ganler]

LGTM. Thanks for the improvement!