PyTorch slice fixes
This merge request has the following changes:
- Bug fix in sadl/layer_slice.h. Modifications for the model's slice layer-specific variables are prevented. Using local variables instead.
- Support onnx to SADL conversion for slice layers from PyTorch.
- Test model has been added to utests/models directory. It is present at utests/models/slice_chw_pytorch.py(creator of onnx model) and utests/models/slice_chw_pytorch.onnx.
- utests/check.sh has been modified for the new slice_chw_pytorch.onnx model.