More flexible conv2d_NCHWc_int8 generic operator.

[cbalint13]

This small PR enable flexible parametrizations for conv2d_NCHWc_int8() int terms of int32_lanes and int8_elements .

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Goal:

• Allow conv2d_NCHWc_int8() to be called with any custom values of int32_lanes and int8_elements
• Such flexibility (custom lane&elements) will be desired for future customizable ISA.

Changes:

1. Make n_elems (default=4) optional parameter instead of static n_elems = 4 hardcode.
2. Default fallback config now emit valid configuration for cases whenint32_lane < int8_num_elements too.
3. All old behavior is kept, so actual Intel or ARM specific intrinsic will benefit the same default values as before.

@anijain2305 @zhiics @vinx13 @ZihengJiang please help me with the review,

Thank You !

[giuseros]

Thanks for the explanation and for the pointers @cbalint13 ! This LGTM

[cbalint13]

Thanks for the explanation and for the pointers @cbalint13 ! This LGTM

@giuseros , Thank you very much !

[giuseros]

I didn't know I had the possibility to approve PRs without being a "reviewer". Apparently yes :)

[vinx13]

@cbalint13 could you try restarting the CI?

[cbalint13]

@cbalint13 could you try restarting the CI?

@vinx13 , done, passed.

Thank You !