fix(dynamo): harden dynamic converter boundaries

[narendasan]

Summary

• clamp finite slice stops against a dynamic runtime axis extent
• keep rank-8-and-higher tensors out of TensorRT partitions at both producer and boundary checks
• compute optimization-profile opt extents as the true midpoint between min and max
• handle unbounded symbolic range limits without calling int() on infinity-like sentinels

Issues

Closes #4607
Closes #4608
Closes #4610
Closes #4611

Tests

• pytest tests/py/dynamo/conversion/test_slice_aten.py tests/py/dynamo/models/test_dynamic_shape_user_bounds.py tests/py/dynamo/partitioning/test_rank_limit.py -n0 (56 passed)
• original reproducers for 🐛 [Bug] slice does not clamp a finite stop on a dynamic axis, baking the literal stop into the engine #4607, 🐛 [Bug] rank-8+ tensors cross the partition boundary and fail in add_input, not at their producer #4608, 🐛 [Bug] optimization-profile opt extent is computed as min + max/2, exceeds max value in certain cases #4610, and 🐛 [Bug] extract_var_range_info calls int() on unbounded range bounds (no lower guard, upper guards only int_oo) #4611 no longer reproduce

Stack

6 of 7. Base: #4631. Follow-up: narendasan/fix-symbolic-converter-parameters.