at-dispatch-v2

# AT_DISPATCH to AT_DISPATCH_V2 Converter

This skill helps convert PyTorch's legacy AT_DISPATCH macros to the new AT_DISPATCH_V2 format, as defined in `aten/src/ATen/Dispatch_v2.h`.

## When to use this skill

Use this skill when:
- Converting AT_DISPATCH_* macros to AT_DISPATCH_V2
- Porting ATen kernels to use the new dispatch API
- Working with files in `aten/src/ATen/native/` that use dispatch macros
- User mentions "AT_DISPATCH", "dispatch v2", "Dispatch_v2.h", or macro conversion

## Quick reference

**Old format:**
```cpp
AT_DISPATCH_ALL_TYPES_AND3(kBFloat16, kHalf, kBool, dtype, "kernel_name", [&]() {
  // lambda body
});
```

**New format:**
```cpp
AT_DISPATCH_V2(dtype, "kernel_name", AT_WRAP([&]() {
  // lambda body
}), AT_EXPAND(AT_ALL_TYPES), kBFloat16, kHalf, kBool);
```

## Key transformations

1. **Reorder arguments**: `scalar_type` and `name` come first, then lambda, then types
2. **Wrap the lambda**: Use `AT_WRAP(lambda)` to handle internal commas
3. **Expand type groups**: Use `AT_EXPAND(AT_ALL_TYPES)` instead of implicit expansion
4. **List individual types**: Add extra types (kHalf, kBFloat16, etc.) after expanded groups
5. **Add include**: `#include <ATen/Dispatch_v2.h>` near other Dispatch includes

## Instructions

### Step 1: Add the Dispatch_v2.h include

Add the v2 header near the existing `#include <ATen/Dispatch.h>`:

```cpp
#include <ATen/Dispatch.h>
#include <ATen/Dispatch_v2.h>
```

Keep the old Dispatch.h include for now (other code may still need it).

### Step 2: Identify the old dispatch pattern

Common patterns to convert:

- `AT_DISPATCH_ALL_TYPES_AND{2,3,4}(type1, type2, ..., scalar_type, name, lambda)`
- `AT_DISPATCH_FLOATING_TYPES_AND{2,3}(type1, type2, ..., scalar_type, name, lambda)`
- `AT_DISPATCH_ALL_TYPES_AND_COMPLEX_AND{2,3}(type1, ..., scalar_type, name, lambda)`
- `AT_DISPATCH_FLOATING_AND_COMPLEX_TYPES_AND{2,3}(type1, ..., scalar_type, name, lambda)`

### Step 3: Map the old macro to type groups

Identify which type g