Add extra signal support to 2DGS rasterization

[yunusstalha]

#998

This PR adds auxiliary per-Gaussian signal rendering support to rasterization_2dgs(), mirroring the existing extra_signals API in the 3DGS rasterizer.

The motivation is to allow 2DGS users to render learned features, semantic logits, supervision targets, or other auxiliary Gaussian-attached signals through the same visibility ordering and alpha compositing path as the normal 2DGS render, while keeping those auxiliary outputs separate from RGB/depth outputs.

Previously, users could only approximate this by packing extra channels into colors, which conflates appearance with auxiliary outputs and makes it harder to preserve a clean render contract. This PR adds a dedicated path instead.

Main Changes

• Adds extra_signals and extra_signals_sh_degree arguments to rasterization_2dgs().

• Supports post-activation auxiliary signal layouts:

  • [..., N, E]
  • [..., C, N, E]

• Supports SH-evaluated auxiliary signals with layout:

  • [N, K, E]

• Renders auxiliary signals through the same 2DGS alpha compositing path as colors.

• Stores the auxiliary render separately in: 'meta["render_extra_signals"]

• Keeps RGB/depth outputs separate from auxiliary outputs, including for RGB+D, D, and ED render modes.

• Adds explicit metadata describing the auxiliary signal receipt/round-trip contract:

meta["extra_signal_layout"]
meta["extra_signal_channels"]
meta["extra_signals_sh_degree"]
meta["extra_signal_source"]
meta["extra_signal_compositing"]

• Adds channel chunking support for large combined color/depth/signal feature tensors.

Testing
This PR adds focused 2DGS tests covering:

• Numeric compatibility when extra_signals=None.
• Metadata and shape checks for rendered auxiliary signals.
• Per-Gaussian and per-camera signal layouts.
• Packed rasterization mode.
• RGB, RGB+D, D, and ED render modes.
• SH-based auxiliary signals.
• Channel chunking.
• Gradient flow through auxiliary signal values and Gaussian parameters.
• A tiny deterministic stats fixture to help detect future changes to compositing or visibility semantics.