[TOPI] Numpy consistency: always broadcast binary op. (#1321)

docs/Doxyfile

docs/api/python/topi.rst

nnvm/python/nnvm/top/nn.py

nnvm/python/nnvm/top/tensor.py

nnvm/src/top/tensor/broadcast.cc

nnvm/src/top/tensor/elemwise.cc

python/tvm/expr.py

python/tvm/generic.py

tests/python/unittest/test_lang_tensor_overload_op.py

topi/include/topi/broadcast.h

topi/include/topi/elemwise.h

topi/include/topi/nn/l2_normalize.h

topi/include/topi/nn/local_response_norm.h

topi/python/topi/broadcast.py

topi/python/topi/cpp.py

topi/python/topi/generic_op_impl.py

topi/python/topi/math.py

topi/python/topi/tensor.py

topi/src/topi.cc

topi/tests/python/test_topi_broadcast.py