* add variable_length_memory_efficient_attention
* update variable_length_memory_efficient_attention unittest
* update variable_length_mem_eff_attn's docs and unittest
* update variable_length_mem_eff_attn's docs
* Update test_variable_length_memory_efficient_attention.py
* Update variable_length_memory_efficient_attention.cu
* fix codestyle
* fix variable_length_fmha's docs and unittest
* fix variable_length_fmha's docs