Add NHWC support in Portable Mean

[digantdesai]

hmm, NHWC support might not be there for portable.mean

executorch/kernels/portable/cpu/op_mean.cpp

Lines 37 to 40 in 54b8176

	ET_KERNEL_CHECK(
	ctx, tensors_have_same_dim_order(in, out), InvalidArgument, out);
	ET_KERNEL_CHECK(ctx, tensor_is_default_dim_order(in), InvalidArgument, out);

Originally posted by @digantdesai in #19139