[OpenReg] Add DeviceTypeTestBase example with instantiate_device_type_tests

### 🚀 The feature, motivation and pitch

## Summary
Add a working example of `DeviceTypeTestBase` and `instantiate_device_type_tests()` to OpenReg, providing a documented reference for backend authors on how to implement device-agnostic tests.

## Background

As outlined in [#158917](https://github.com/pytorch/pytorch/issues/158917), OpenReg aims to serve as the official reference implementation for new device integration into PyTorch. A key part of this is demonstrating how to use PyTorch's standardized testing infrastructure.

[DeviceTypeTestBase](https://github.com/pytorch/pytorch/blob/main/torch/testing/_internal/common_device_type.py#L317) allows writing tests once that automatically generate `_cpu`, `_cuda`, `_openreg` (and other device) variants.

Parent issue: [169597](https://github.com/pytorch/pytorch/issues/169597)
## Proposed Tests

The example should include tests for:

- Basic tensor creation
- `test_basic_tensor_creation` - Create tensors on device
- `test_empty_tensor` - `torch.empty()` semantics
- `test_zeros_ones` - `torch.zeros()` and `torch.ones()`
- Core elementwise operations
- Dtype support and propagation
- Device transfer/copy correctness
- Device-specific skip patterns
- ...

## Expected Outcome

- Tests are device-agnostic and generate `_cpu`, `_cuda`, `_openreg` variants automatically
- Validates that OpenReg correctly integrates with PyTorch's test infrastructure

## CC

@parsshar-RH @ahkush @groenenboomj

### Alternatives

_No response_

### Additional context

_No response_