Add Sendable closures and 3-parameter VJP support to MLXCustomFunction

[shubhransh-gupta]

Description

Following discussion on #475:

1. Make Forward and VJP components conform to @Sendable, ensuring custom functions are pure and safe across concurrency contexts.
2. Make CustomFunction return @Sendable ([MLXArray]) -> [MLXArray].
3. Add a 3-parameter overload for VJP:

   public func VJP(
       _ f: @escaping @Sendable (_ primals: [MLXArray], _ cotangents: [MLXArray], _ outputs: [MLXArray]) -> [MLXArray]
   ) -> MLXCustomFunctionComponent

   This passes through the forward outputs already provided by the underlying C closure (mlx_closure_custom_new_func_payload) without needing extra C++/Cmlx changes.
4. Maintain full backward compatibility for the 2-parameter VJP (_ primals: [MLXArray], _ cotangents: [MLXArray]) signature.
5. Add unit tests in TransformTests covering 2-parameter VJP, 3-parameter VJP using outputs, and cross-task Sendability.

Closes #475

[davidkoski]

This needs a swift-format run (the lint failure)