Enable dynamic shapes for foreach operations by default (pytorch#158985)

## Summary

This PR changes the default value of `combo_kernel_foreach_dynamic_shapes` from `False` to `True` in `torch/_inductor/config.py`.

## Context

The `combo_kernel_foreach_dynamic_shapes` configuration was introduced in PR pytorch#134477 (August 2024) to support dynamic shapes for foreach and combo kernels. It was initially disabled by default as a conservative approach to avoid disrupting production workflows.

## Why This Change?

After several months of the feature being available and stable, it's time to enable it by default. This improves the user experience for developers using `torch.compile(dynamic=True)` with foreach operations.

### Current behavior:
- Users must manually discover and enable `combo_kernel_foreach_dynamic_shapes`
- Without this flag, foreach operations may fail with dynamic shapes
- This creates friction and confusion

### With this change:
- Foreach operations work seamlessly with dynamic compilation
- No manual configuration needed
- Better "it just works" experience

## Testing

Extensive testing was performed with PyTorch 2.5.0+ and 2.7.1:
- ✅ Various tensor sizes (8, 16, 32, 64, 128)
- ✅ Multiple tensors in operations (tested up to 20)
- ✅ Nested foreach operations
- ✅ Mixed operations (foreach + standard operations)
- ✅ Both CPU and CUDA devices
- ✅ Symbolic shapes with dynamic compilation

## Impact Assessment

- **Performance**: No impact - this only affects compilation behavior
- **Backward Compatibility**: Fully maintained - users can still set to `False`
- **Risk**: Minimal - feature has been stable since August 2024

## References

- Original implementation: PR pytorch#134477 by @qchip
- This completes the feature rollout by making it available by default

Pull Request resolved: pytorch#158985
Approved by: https://github.com/jansel, https://github.com/mlazos

Author: JonSnow1807

test/inductor/test_foreach.py

@@ -645,6 +645,7 @@ def fn(a0, a1, b0, b1):
     @bin_ops
     @torch._dynamo.config.patch("automatic_dynamic_shapes", False)
     @torch._dynamo.config.patch("assume_static_by_default", False)
+    @torch._inductor.config.patch("combo_kernel_foreach_dynamic_shapes", False)
     def test_dynamic_shapes_fallback(self, op):
         def fn(a0, a1, b0, b1):
             return op([a0, a1], [b0, b1])

torch/_inductor/config.py

@@ -677,7 +677,7 @@ def use_autoheuristic(name: str) -> bool:
 # for all except for foreach, 2 - enable for all
 combo_kernel_allow_mixed_sizes = 1
 # Enable dynamic shapes for foreach kernels
-combo_kernel_foreach_dynamic_shapes = False
+combo_kernel_foreach_dynamic_shapes = True
 
 # constant folding on the joint graph
 joint_graph_constant_folding = True