[Quant] make x86 the default quantization backend (qengine)

[Xia-Weiwen]

Summary
Make x86 the default quantization backend (qengine) for X86 CPU platforms.
X86 is a unified quantization backend combining goodness of fbgemm and onednn. For more details please see #83888

Test plan
python test/test_quantization.py

cc @jerryzh168 @jianyuh @raghuramank100 @jamesr66a @vkuzo @jgong5 @leslie-fang-intel @mingfeima @XiaobingSuper @sanchitintel @ashokei @jingxu10

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[malfet]

PR description does not match with the actual implementation - i.e. it makes x86 a default backend for all platforms (not just x86, but ARM as well)

If this quantization backend supposed to work on all CPUs, let's rename it to something else.
If not, please guard the check with #if defined(__x86_64__) || defined(WHATEVER_IT_SHOULD_BE_FOR_WINDOWS)

[Xia-Weiwen]

PR description does not match with the actual implementation - i.e. it makes x86 a default backend for all platforms (not just x86, but ARM as well)

If this quantization backend supposed to work on all CPUs, let's rename it to something else. If not, please guard the check with #if defined(__x86_64__) || defined(WHATEVER_IT_SHOULD_BE_FOR_WINDOWS)

Thanks @malfet. X86 backend is only enabled when fbgemm is enabled. Looks like fbgemm only supports x86_64 platforms, right? In that case, x86 backend is enabled on x86_64 platforms only, and additional guard is not needed.

[malfet]

Thanks @malfet. X86 backend is only enabled when fbgemm is enabled. Looks like fbgemm only supports x86_64 platforms, right? In that case, x86 backend is enabled on x86_64 platforms only, and additional guard is not needed.

Good point, looks good to me then.

[malfet]

@pytorchbot merge

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