[Intel MKL] Fixes for unit test failures

1) Changes in partitioned_function_ops.cc are for passing
   Global OpRegistry as default_registry in PartitionedFunction op

   This fix addresses failure in MKL layout pass when PartitionedFunction
   op calls graph optimization passes. The problem was that the function
   library definition that is used to create function graph and corresponding
   subgraphs after partitioning did not use global OpRegistry as the
   default OpRegistry used for look of operator names. Because of that,
   standard operators such as "Const" were not available to graph passes.

2) Changes in mkl_cpu_allocator.h are to address failure in
   mkl_cpu_allocator_test which was expecting that max_bytes_limits is returned
   via GetStats() in MKLCPUAllocator.