[Bug] [Relax] Inconsistent buffers compute and lv mapped to the same relax var about tvm HOT 3 CLOSED

Looks like this is both a bug in the ToMixedPrecision pass, and a limitation in the well-formed checker. The bug is that the current implementation of ToMixedPrecision implicitly assumes that all tensors are float32 before the pass is applied. As a result, when the annotation MixedPrecisionPolicyKind::kNever (that is, never change the dtype of this operator) is encountered for R.call_tir, it casts the input/output of R.call_tir to float32.

The limitation in the well-formed checker is that the callee in R.call_tir doesn't get inspected to identify this issue when it first occurs, so it only is caught later by FuseTIR. The StructuralEqual() check looks like it was intended to catch a mismatched TIR buffer shape, but serendipitously caught the mismatched dtype between the output of fused_layer_norm_cast and the input of conv2d_cast_relu.

To fix this, I think the ToMixedPrecision pass will need updated behavior for kNever. Where currently, when an operator with mixed-precision policy of kNever will cast all inputs to float32, it should instead cast all inputs to the same dtype as they had prior to ToMixedPrecision.

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@Lunderberg Thanks for your deep investigation and quick fixing.

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And should be fixed in #17263

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