Reduce memory usage for TreeEnsemble operators (#14670)
### Description
The onnx file is about 5Mb for a lightgbm model with 500 trees.
onnxruntime uses additional 10Mb to compute the inference and keeps the
onnx structure. This PR reduces the memory usage by almost 50%. The
memory used by the onnx node could be freed if there is no optimized
graph to save but that's not covered by this PR.
### Motivation and Context
Reduce memory usage.
