Refine porting x86 NCHWc conv to AutoTVM (#1993)

nnvm/python/nnvm/top/nn.py

python/tvm/autotvm/task/dispatcher.py

python/tvm/autotvm/task/space.py

python/tvm/autotvm/task/task.py

python/tvm/autotvm/task/topi_integration.py

topi/python/topi/generic/nn.py

topi/python/topi/hls/nn.py

topi/python/topi/intel_graphics/conv2d.py

topi/python/topi/nn/conv2d.py

topi/python/topi/x86/conv2d.py

topi/python/topi/x86/conv2d_avx_1x1.py

topi/python/topi/x86/conv2d_avx_common.py

topi/recipe/conv/test_conv_int8_intel.py

topi/tests/python/test_topi_conv2d_NCHWc.py

tutorials/autotvm/tune_nnvm_x86.py