Created by: qingqing01

fix #4629 (closed) fix #4675 (closed)

• Finish the implementation kernel, but the code style will be refined later.
• Finish the forward implementation and the unit testing.
• Support reversed LSTM.
• Support peephole connections by default.
• TODO in next PR:
  • Backward implementation and it's unit testing.
  • Improve code about the kernel implementation in math/detail.
  • Support to disable peephole connections.
  • Enable initial hidden state and initial cell state.
  • Now use a SequenceToBatch functor to reorganize and sort the input sequence, I'll try to use TensorArray in the future.

Directory structure:

├── lstm_op.cc
├── lstm_op.cu
├── lstm_op.h
├── math
│   ├── lstm_compute.cc
│   ├── lstm_compute.cu
│   ├── lstm_compute.h [One batch computing functor for lstm.]
│   ├── sequence2batch.cc
│   ├── sequence2batch.cu
│   ├── sequence2batch.h [Reorganize and sort the input sequence,  try to use `TensorArray` in the future.]
│   ├── detail [The code in this directory will be refined later.]
│   │   ├── CMakeLists.txt
│   │   ├── hl_activation_functions.h
│   │   ├── hl_avx_functions.cc
│   │   ├── hl_avx_functions.h
│   │   ├── hl_cpu_functions.cc
│   │   ├── hl_functions.h  [Activations for cpu, gpu and avx]
│   │   ├── hl_gpu_functions.h 
│   │   ├── lstm_cpu_kernel.h [CPU implenmentation kernel for one batch and one sequence.]
│   │   ├── lstm_gpu_kernel.h [GPU implenmentation kernel for one batch.]
│   │   └── lstm_kernel.h