[LTC] Code-gen exp, elu*, and log_sigmoid*
Summary:
This commit adds code-gen support for the following ops:
- exp
- elu
- elu_backward
- log_sigmoid_forward
- log_sigmoid_backward
It also improves the unit-test AtenLtcTsTensorTest.TestLogSigmoid and
adds unit test AtenLtcTsTensorTest.LogSigmoidForward.
Test Plan:
test/cpp/build/test_ptltc --gtest_filter=AtenLtcTsTensorTest.TestExp:AtenLtcTsTensorTest.TestElu*:AtenLtcTsTensorTest.TestLogSigmoid*
