Porting schedules (except convolutions) to C++ (#763)

* Ported injective schedules to C++. Added some elementwise ops.

* Fix lint errors

* Added reduction ops and schedules

* Fix lint errors

* Fix lint errors

* Fix lint errors

* Added transform ops

* Fix lint errors

* Fix lint errors

* Added softmax, log_softmax, leaky_relu and flatten ops.
Fixed issue where TVM_DECLARE_INTRIN_UNARY used the PureExtern flag
instead of PureIntrinsic.
Added softmax CUDA schedule.

* Fix lint

* Fix lint

* Added binary_dense, batch_norm_inference, dense, dilate, scale_shift_*,
global_pool and pool ops.
Extended pad to allow specifying pad_value.
Fixed issue where pad would throw if padding was zero in all dimensions.

* Fix lint

* Fix lint

* Added CUDA schedules for dense, pool and global_pool

* Added extern schedules for generic and CUDA

* Fix lint

* Added x86 binary schedules

* Fix lint

* Added rocm dense schedule. Added rocBLAS and cuBLAS support to dense ops

* Added pow ops. Added x86 default and injective schedules

* Fix lint

* Fix lint

* Fix lint

* Fix lint

* Fix lint

* Fix indent

* Removed schedules directory

* Changed left_shift, right_shift to operators. Changed pad_value in pad() to remove pointer usage

* Fixed usage of pad in nn/pooling.h. Fixed declaration of operator>>

* Fixed comments for shift operators

* Added comments to utility functions

* Added TOPI C++ library, exporting broadcast_add op

* Fix lint

* Share libinfo.py with TVM

* Fix lint

* Add other broadcast ops

* Fix lint

* Fix imports in topi

* Fix lib names

* Fixed build issue where windows builds don't apply correct definitions

* Removed TVM_EXPORTS from topi library

* Attempted CI build fix

* Add topi lib to tvm_multilib

* Fix Jenkinsfile

* Added TOPI build target to Makefile

* Fix nn op namespaces.

* Fix lint

* Renamed TOPI lib to libtvm_topi

* Removed _ffi/base.py

* Remove _ffi from topi, now shared with tvm.

* Make libtvm_topi loading optional

* Fix compiler warnings

* Fix lint

* Fix lint

* Fix lint

* Fix build error by making new libs argument to Target optional

* Added C++ Target type interop. Added registration of remaining C++ ops and schedules. Added test of broadcast ops

* Fix lint

* Fix lint

* Fix compile error

* Fix compiler warnings

* Fix compiler warnings

* Fixed int vector interop. Fixed argmin incorrectly invoking argmax. Fixed corner case in default schedules of attempting to fuse 0 length axes. Added tests for reduce ops.

* Refactored reduce builders

* Fixed typos in topi.cc. Added basic test.

* Fixed padding size error. Added dense, dilate, pooling tests

* Fixed issue where clip would output a different dtype to the input. Added split_sections op to cover the other mode of the python split op. Added tests.

* Changed extension type numbers to avoid clash with NNVM

* Fix lint

* Fix compiler warnings

* Removed use of std::vector from the public TOPI API

* Fix lint

* Add TOPI C++ tests to CI

* Fixed detail namespacing. Improved comments.