DickJC123 and eric-haibin-lin CudnnFind() usage improvements (#12804)
* Add mx.context.gpu_memory_info() to python api for flexible tests.

* Add test_gluon_gpu.py:test_large_models to show cudnnFind headroom issue.

* Output model sizes tried by test_gluon_gpu.py:test_large_models.

* Fix perl interface to MXGetGPUMemoryInformation.

* Increase difficulty of test_gluon_gpu.py:test_large_models.

* Forgot a file in fix for perl.

* Modify test to pass on no-cudnn CI runner.

* Mutex algo reg updates, serialize cudnnFind calls.

* Fix for cudnnFind memory headroom issue.

* Fix cpplint.

* Respond to reviewers comments.

* Guard against improper MXNET_GPU_MEM_LARGE_ALLOC_ROUND_SIZE values.

* Fix potentially unassigned var.
Latest commit ee5f699 Oct 26, 2018