`keepDimensions`  of reduction ops

XNNPACK can't support keepDimensions = true for ReduceMean.
This issue was raised by @huningxin in WebNN [Chromium CL review](https://chromium-review.googlesource.com/c/chromium/src/+/4697704/comment/4b154e0f_984922ff/). Thanks Ningxin!