[quant] Add default symmetric qconfig for qnnpack (#74396)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74396
# New qconfig `default_symmetric_qnnpack_qconfig`
Returns a qconfig with signed activation and symmetric weights with range restrictions. Also adds per_channel variant for the same.
## Restrictions on weights
Restrictions on weights include,
1. weight zero point is force zero. and
2. weight 8-bit signed quantized value are limited to [-127, +127] excluding the value +128.
This is driven, in part, by the desire to achieve better performance by XNNPACK ops.
## qengine/backend = `qnnpack` and XNNPACK ops
Qconfig returned by this function allows us to use faster XNNPACK quantized ops for CPUs w/ said restrictions. Although we are using XNNPACK ops the qengine is still `qnnpack`, and there are no plans to introduce a new qengine for XNNPACK ops. Support to use XNNPACK ops with asymmetric (returned by get_default_qconfig()) qconfig is WIP.
## Updated EPS value:
* From PyTorch:
eps:
```
>>> import torch
>>> torch.finfo(torch.float32).eps
1.1920928955078125e-07
>>> torch.finfo(torch.float32).eps.hex()
'0x1.0000000000000p-23'
```
All scale values are float32 and `scale = max(scale, eps)`
* Requirement from XNNPACK
For both fp32 as well as rndnu requantization schema, `0x1p-32 <= requantization_scale < 256.0`
Where, requantization_scale = (input_scale * kernel_scale) / (output_scale)
* New minimum allowed scale value
With current float32 eps (=0x1p-23) as minimum, xnnpack lower bound is the problem. We haven’t observed upper bound issues so far with assuming the max scale value of 256. So focusing on the lower bound, to cover all possible cases of requantization value, conservatively, we must have the minimum possible requantization scale value such that,
```
minimum_requantization_value = xnnpack_lower_threshold
input_scale * kernel_scale / output_scale = 0x1p-32
min_scale_value * min_scale_value / max_scale_value = 0x1p-32
min_scale_value * new_eps / 256 = 0x1p-32
min_scale_value**2 = 0x1p-24
min_scale_value = 0x1p-12
```
With `scale_value >= 0x1p-12`, we should be able to avoid the lower threshold on requantization scale by xnnpack kernels.
Obviously this is a very unlikely to happen. So practically, we should be get away with much smaller value than `0x1p-12` as EPS, but it is not easy to choose a smaller value empirically.
* Impact on accuracy is unclear as of writing this.
Reviewed By: kimishpatel
Differential Revision: D34625300
fbshipit-source-id: 005e6757ed1185b3940b58ac55246cba8b267828
(cherry picked from commit 61ed1a2a308a1792ccbfc316153a6dc39798f02a)
