Fix illegal memory acess issue for CUDA versionn of SplitByLengths operator.
Summary:
1. Fix illegal memory access issue for SplitByLengths operator in the CUDA context.
2. Add support to scaling lengths vector for SplitByLengths operator.
3. Add support to test SplitByLengths operator in the CUDA context.
Example for SplitByLengths operator processing scaling lengths vector:
value vector A = [1, 2, 3, 4, 5, 6]
length vector B = [1, 2]
after execution of SplitByLengths operator,
the output should be [1,2] and [3,4,5,6]
Test Plan: buck test mode/dev-nosan caffe2/caffe2/python/operator_test:concat_split_op_test
Reviewed By: kennyhorror
Differential Revision: D22780307
fbshipit-source-id: c5ca60ae16b24032cedfa045a421503b713daa6c
