[tosa] Fix legalization of tfl.slice for -1 size
Errors on the ARM CI seem to be:
```
ERROR: /workspace/bazel_pip/tensorflow/dtensor/python/tests/BUILD:22:15: in py_library rule //bazel_pip/tensorflow/dtensor/python/tests:test_util: target '//tensorflow/dtensor/python:api' is not visible from target '//bazel_pip/tensorflow/dtensor/python/tests:test_util'. Check the visibility declaration of the former target if you think the dependency is legitimate
(...)
ERROR: command succeeded, but not all targets were analyzed
```

All tests built passed though:
`Executed 1359 out of 1359 tests: 1359 tests pass.` Ping?
TOSA does not support -1 size so convert to the equivalent size - begin behaviour.