Summary:

This should have been part of
#24720, which added
`QuantAbsorptionPass` with the other fused_quant graph passes. It was left out
of that PR by mistake.

By default, `QuantAbsorptionPass` now only runs the rewrite that is never less
accurate. The docstring explains why the other two modes are opt-in.

- Output side (unchanged, always on): the producer writes its output directly
  in the downstream quantize's qparams. The value still ends up on that grid,
  rounded once instead of twice, so each element is at least as close to the
  float result. It isn't bit-exact with the q/dq graph, though.
- Input side (now opt-in, `absorb_into_inputs`): the consumer reads values on
  the dequantize's grid instead of the quantize's. Dropping the second rounding
  is more accurate on average, but not for every value.
- Forked dequantize (already opt-in, `absorb_with_fork`): its float users see
  values on the quantize's grid instead of the producer's, which can be
  coarser. Only the docstring changes.

With input-side absorption off, a pair that only the input side would remove
is left to `ReplaceDequantQuantWithRequantize`, which turns it into a
`fused_quant.requantize`.

Differential Revision: D130466673

Summary:

This should have been part of
#24720, which added
`QuantAbsorptionPass` with the other fused_quant graph passes. It was left out
of that PR by mistake.

By default, `QuantAbsorptionPass` now only runs the rewrite that is never less
accurate. The docstring explains why the other two modes are opt-in.

- Output side (unchanged, always on): the producer writes its output directly
  in the downstream quantize's qparams. The value still ends up on that grid,
  rounded once instead of twice, so each element is at least as close to the
  float result. It isn't bit-exact with the q/dq graph, though.
- Input side (now opt-in, `absorb_into_inputs`): the consumer reads values on
  the dequantize's grid instead of the quantize's. Dropping the second rounding
  is more accurate on average, but not for every value.
- Forked dequantize (already opt-in, `absorb_with_fork`): its float users see
  values on the quantize's grid instead of the producer's, which can be
  coarser. Only the docstring changes.

With input-side absorption off, a pair that only the input side would remove
is left to `ReplaceDequantQuantWithRequantize`, which turns it into a
`fused_quant.requantize`.

Reviewed By: DrJessop

Differential Revision: D130466673