Split solver/momentum_ik.cpp into separate autograd files (#1082)

Summary:
Pull Request resolved: #1082

This diff refactors `arvr/libraries/pymomentum/solver/momentum_ik.cpp` (originally 1505 LOC) by extracting the autograd function definitions into a separate `momentum_ik_autograd.cpp` / `.h` pair.

**Changes:**
- Created `momentum_ik_autograd.h` (265 LOC) containing:
  - `TensorMppcaModel` struct
  - `extractMppcaModel()` function declaration
  - `createIKProblem<T>()` template function declaration
  - `IKSolveFunction<T>`, `GradientFunction<T>`, `ResidualFunction<T>`, `SequenceIKSolveFunction<T>` class declarations
  - `IKProblemAutogradFunction<T, IKFunction>` struct declaration

- Created `momentum_ik_autograd.cpp` (1002 LOC) containing implementations of all the above

- Simplified `momentum_ik.cpp` (now 539 LOC) to contain only:
  - `applyIKProblemAutogradFunction` helper (in anonymous namespace)
  - Public pybind11 registration functions: `solveBodyIKProblem`, `computeGradient`, `computeResidual`, `computeJacobian`, `solveBodySequenceIKProblem`, `transformPose`

This resolves the issue of a single file mixing autograd function definitions with pybind11 binding registration, making the code more maintainable and reducing `momentum_ik.cpp` well below the 1500 LOC target.

Reviewed By: cdtwigg

Differential Revision: D95089972

fbshipit-source-id: 578014cbcbca6908a1ffc44fa5edffd147dffc22

Author: cstollmeta

pymomentum/cmake/build_variables.bzl

@@ -179,10 +179,12 @@ diff_geometry_sources = [
 
 solver_public_headers = [
     "solver/momentum_ik.h",
+    "solver/momentum_ik_autograd.h",
 ]
 
 solver_sources = [
     "solver/momentum_ik.cpp",
+    "solver/momentum_ik_autograd.cpp",
     "solver/solver_pybind.cpp",
 ]