pooling test fail when using MIOPEN as engine

[petrex]

miopen poolinf test fail.

Trying example: test_global_avg_pool_nchw(self=<__main__.TestPooling testMethod=test_global_avg_pool_nchw>, op_type='AveragePool2D', sz=2, batch_size=2, engine='MIOPEN', gc=device_type: 4, dc=[, device_type: 4])
*** Aborted at 1521584246 (unix time) try "date -d @1521584246" if you are using GNU date ***
PC: @     0x7fdf717181b1 miopen::PoolingDescriptor::GetForwardOutputDim()
*** SIGSEGV (@0x0) received by PID 62799 (TID 0x7fdfa3e49700) from PID 0; stack trace: ***
    @     0x7fdfa3a39390 (unknown)
    @     0x7fdf717181b1 miopen::PoolingDescriptor::GetForwardOutputDim()
    @     0x7fdf7158e838 miopenGetPoolingForwardOutputDim
    @     0x7fdf95e7e807 caffe2::MIOPENPoolOp::DoRunWithType<>()
    @     0x7fdf95e71f56 caffe2::MIOPENPoolOp::RunOnDevice()
    @     0x7fdf94a661fe caffe2::Operator<>::Run()
    @     0x7fdf76cf6732 caffe2::Workspace::RunOperatorOnce()
    @     0x7fdf9754ab4a _ZZN6caffe26python16addGlobalMethodsERN8pybind116moduleEENKUlRKNS1_5bytesEE26_clES6_.isra.2495.constprop.2527
    @     0x7fdf9754ade5 _ZZN8pybind1112cpp_function10initializeIZN6caffe26python16addGlobalMethodsERNS_6moduleEEUlRKNS_5bytesEE26_bJS8_EJNS_4nameENS_5scopeENS_7siblingEEEEvOT_PFT0_DpT1_EDpRKT2_ENUlRNS_6detail13function_callEE1_4_FUNESQ_
    @     0x7fdf9756ad43 pybind11::cpp_function::dispatcher()
    @           0x4c1d50 PyEval_EvalFrameEx
    @           0x4b9ab6 PyEval_EvalCodeEx
    @           0x4c1e6f PyEval_EvalFrameEx
    @           0x4b9ab6 PyEval_EvalCodeEx
    @           0x4c16e7 PyEval_EvalFrameEx
    @           0x4b9ab6 PyEval_EvalCodeEx
    @           0x4c16e7 PyEval_EvalFrameEx
    @           0x4b9ab6 PyEval_EvalCodeEx
    @           0x4d55f3 (unknown)
    @           0x4a577e PyObject_Call
    @           0x4bed3d PyEval_EvalFrameEx
    @           0x4b9ab6 PyEval_EvalCodeEx
    @           0x4c1e6f PyEval_EvalFrameEx
    @           0x4b9ab6 PyEval_EvalCodeEx
    @           0x4d55f3 (unknown)
    @           0x4a577e PyObject_Call
    @           0x4bed3d PyEval_EvalFrameEx
    @           0x4b9ab6 PyEval_EvalCodeEx
    @           0x4c1e6f PyEval_EvalFrameEx
    @           0x4b9ab6 PyEval_EvalCodeEx
    @           0x4c1e6f PyEval_EvalFrameEx
    @           0x4b9ab6 PyEval_EvalCodeEx
^C[1]    62799 segmentation fault (core dumped)  python -m caffe2.python.operator_test.pooling_test

[petrex]

We need to address test issue to validate MIOPEN pooling op.

for example :

• MIOPEN only support 2D pooling (Avg and Max)
• MIOPEN only support NCHW mode.

[petrex]

fixed in #76