[CPU] Add ops for float8 linear

[Xia-Weiwen]

Summary
This is part of a previous PR #2505 since the original is too big.
This PR adds two ops for float8 linear on CPU, one for weight packing and the other for computation.

• float8_linear_prepack_cpu
• float8_linear_cpu

They will be used for float8 tensor subclass in the future.

Test plan

pytest -sv test/test_ops.py -k test_float8_linear_cpu

[pytorch-bot]

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As of commit fd3d6b5 with merge base 8e2ca35 ():
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[Xia-Weiwen]

CC @mingfeima This PR is a copy of the op implementation of the previous PR #2505 because the original PR is too big. Thanks.

[mingfeima]

LGTM

[jcaip]

Sorry @Xia-Weiwen @jerryzh168 but this is breaking some internal tests, going to revert: https://www.internalfb.com/tasks/?t=239795912

It looks to be complaining about a warning with the switch statement:

fbcode/pytorch/ao/torchao/csrc/cpu/aten_kernels/float8_linear.cpp:588:3: error: switch condition has boolean value [-Werror,-Wswitch-bool]
  588 |   AT_DISPATCH_LINEAR_KERNEL(output_dtype, cpublas_can_pack, act_quant_mode, wei_quant_mode, [&](){
      |   ^                                       ~~~~~~~~~~~~~~~~
fbcode/pytorch/ao/torchao/csrc/cpu/aten_kernels/utils.h:100:5: note: expanded from macro 'AT_DISPATCH_LINEAR_KERNEL'
  100 |     AT_DISPATCH_BOOL(                                                                   \
      |     ^
  101 |         CAN_PACK, "cpublas_can_pack", can_pack,                                         \
      |         ~~~~~~~~
fbcode/pytorch/ao/torchao/csrc/cpu/aten_kernels/utils.h:84:5: note: expanded from macro 'AT_DISPATCH_BOOL'
   84 |     switch (VALUE) {                                         \
      |     ^       ~~~~~