Reduced default number of threads in reduction kernels for Metal.
Default code generation generated thread block with the following size:
32x32x1. With this size number of threads per threadgroup was equal to
1024 (32 * 32 * 1). Sometimes device doesn't have enough resources and
in this case we will get an exception that the block size is greater
than value of maxTotalThreadsPerThreadgroup.
To prevent such situation we decrease default number of threads. With
this fix every model should work with default codegen and auto-tuning or
auto-scheduling will select the optimal number of threads.
Egor Churaev
8ea6a30925