* enhance pad.* error message, test=develop

* update the ut precision of pad pad2d pad_constant_like from fp32 to fp64, test=develop

* Remove self-set accuracy parameters of op tests: max_relative_error
test=develop

* Remove self-set accuracy parameters of op tests: max_relative_error
test=develop

* Remove self-set parameters of op tests: max_relative_error
test=develop

* fix input shape of op tests for elementwise_sub,
gather, pad2d, transpose, softmax, scale,
elementwise_max, hierarchical_sigmoid, reshape2,
sign, squeeze, reduce_sum, sum, squeeze2,
unsqueeze, unsqueeze2, cast, reverse

test=develop

* fix cast, elementwise_mul, gather, scale, sign,
softmax, transpose

test=develop

* Make pad2d support for variable paddings.
test=develop

* Rename get_paddings and add inline modifier.
test=develop

* Fix comments.

* Add pad2d op.

* Add unitest and python api.

* Fix cuda op kernel.

* Fix python api.

* Fix python api.

* Update API.spec.

* Fix python api