[PT2E][Quant] Support specifying None for obs_or_fq_ctr in target_dtype_info (#99071)
It is cleaner for quantizer to say what does not need observation instead of
putting fp32 observers. This diff add support for that by checking if
target_dtype_info contains none for specific observers and if so skip inserting
observers for those.
Differential Revision: [D44971357](https://our.internmc.facebook.com/intern/diff/D44971357/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/99071
Approved by: https://github.com/jerryzh168
