Arm backend: Replace asserts with ValueError for slicing constraints (#10748)

The existing code used assertions to validate slicing parameters (e.g.,
size must be positive and within dimension limits). This has been
changed to raise ValueError with descriptive error messages to provide
clearer feedback and robustness for invalid inputs when using
aten.slice_copy.Tensor.

Signed-off-by: Sebastian Larsson <sebastian.larsson@arm.com>