Add support for absolute opeartion (#1406)

docs/api/python/intrin.rst

docs/api/python/topi.rst

docs/nnvm_top.rst

include/tvm/ir_operator.h

nnvm/python/nnvm/top/tensor.py

nnvm/src/top/tensor/elemwise.cc

nnvm/tests/python/compiler/test_top_level3.py

python/tvm/intrin.py

src/api/api_ir.cc

src/codegen/intrin_rule_cuda.cc

src/codegen/intrin_rule_metal.cc

src/codegen/intrin_rule_opencl.cc

src/codegen/llvm/intrin_rule_llvm.cc

src/codegen/llvm/intrin_rule_nvptx.cc

src/codegen/llvm/intrin_rule_rocm.cc

src/codegen/spirv/intrin_rule_spirv.cc

topi/include/topi/elemwise.h

topi/python/topi/math.py

topi/tests/python/test_topi_math.py