at-dispatch-v2

Convert legacy AT_DISPATCH macros to AT_DISPATCH_V2 format for modern dispatch in ATen C++ code.

• Converts AT_DISPATCH_* macros to AT_DISPATCH_V2
• Applies v2 dispatch API in ATen kernel files and native operators
• Supports CUDA kernels and native operator implementations
• Uses Dispatch_v2.h for new type group dispatch

• Reorder args: dtype, name, lambda, types
• Wrap lambda with AT_WRAP to handle internal commas
• Use AT_EXPAND() for type groups (not individual types)
• Add #include <ATen/Dispatch_v2.h> near Dispatch.h
• Map old macros to v2 type groups (e.g., AT_ALL_TYPES)
• Must keep old Dispatch.h include for backward compatibility
• Must not expand individual types with AT_EXPAND()
• Must verify all transformation rules are applied

1. 1.Add Dispatch_v2.h include near Dispatch.h
2. 2.Identify old macro pattern (e.g., ALL_TYPES_AND3)
3. 3.Map to v2 type group (e.g., AT_EXPAND(AT_ALL_TYPES))
4. 4.Extract individual types (e.g., kBFloat16, kHalf)
5. 5.Apply AT_DISPATCH_V2 with correct order and AT_WRAP
6. 6.Verify all rules: order, AT_WRAP, no AT_EXPAND on types