[ONNXGraph] delete_orphaned_node_branches utility function

[bfineran]

utility function that deletes all node branches in an ONNX graph that do not lead to a model output
necessary for the latest LLM graph transformations as rewiring the graphs to take positions and causal masks directly leaves the graphs with orphaned node branches

example_usage:

import onnx
from sparseml.onnx.utils import ONNXGraph

model = onnx.load(PATH_TO_MODEL)
graph = ONNXGraph(model)

graph.delete_orphaned_node_branches()

test_plan:
@dbogunowicz to test with LLMs story. will verify that extraneous nodes are deleted and model correctness remains