Implement BiRefNet and HAT Sharper models with background removal capabilities

- Added validation documentation for AI model tiers.
- Introduced `background.py` for background removal logic, including inference functions for alpha and foreground extraction.
- Created `onnx_birefnet.py` to handle BiRefNet model sessions and alpha inference.
- Developed tests for background models, ensuring proper dispatch and session reuse.
- Added tests for BiRefNet ONNX model to validate alpha output and error handling for invalid values.
- Implemented model export scripts for BiRefNet and HAT Sharper, ensuring proper weight compression and licensing.
- Created `deform_sampling.py` for deformable convolution operations used in BiRefNet.
- Added utility functions for compressing ONNX weights without altering graph arithmetic.
- Introduced a reference plane node for depth field adjustments in geometry processing.

tools/nodes/common/reference_plane.py

@@ -0,0 +1,31 @@
+"""Build the direction-adjusted reference depth field."""
+from .nodes import math_node
+
+
+def reference_plane_depth(group, direction, reference_depth):
+    nodes = group.nodes
+    links = group.links
+    normal = nodes.new("ShaderNodeVectorMath")
+    normal.operation = "NORMALIZE"
+    links.new(direction, normal.inputs[0])
+    normal_components = nodes.new("ShaderNodeSeparateXYZ")
+    links.new(normal.outputs["Vector"], normal_components.inputs["Vector"])
+    position = nodes.new("GeometryNodeInputPosition")
+    position_components = nodes.new("ShaderNodeSeparateXYZ")
+    links.new(position.outputs["Position"], position_components.inputs["Vector"])
+    x_term = math_node(nodes, "MULTIPLY")
+    links.new(position_components.outputs["X"], x_term.inputs[0])
+    links.new(normal_components.outputs["X"], x_term.inputs[1])
+    y_term = math_node(nodes, "MULTIPLY")
+    links.new(position_components.outputs["Y"], y_term.inputs[0])
+    links.new(normal_components.outputs["Y"], y_term.inputs[1])
+    point_dot = math_node(nodes, "ADD")
+    links.new(x_term.outputs[0], point_dot.inputs[0])
+    links.new(y_term.outputs[0], point_dot.inputs[1])
+    divide = math_node(nodes, "DIVIDE")
+    links.new(point_dot.outputs[0], divide.inputs[0])
+    links.new(normal_components.outputs["Z"], divide.inputs[1])
+    plane = math_node(nodes, "ADD")
+    links.new(reference_depth, plane.inputs[0])
+    links.new(divide.outputs[0], plane.inputs[1])
+    return plane.outputs[0]