Improve TreeNodeElementId hash function (#16459)
### Description
This PR improves `TreeNodeElementId` hash function by employing [Elegant
Pairing function](http://szudzik.com/ElegantPairing.pdf). In few works,
Elegant Pairing function maps two non−negative integers to a
non−negative integer that is uniquely associated with that pair. This
drastically reduces the collision and therefore reduces the time
required to create a session in order to use a large tree ensemble
model.
### Motivation and Context
We use ONNX runtime to serve our models as part of Triton backend. We
noticed that it was taking around 2 minutes to load a model which is a
large tree ensemble model (around 5k trees with around 3 millions nodes
in total). After investigating the issue, it was clear that the
`TreeNodeElementId` hash function wasn't being able to map keys to
buckets of C++ `unordered_map` without a significant amount of
collisions (in same cases 700 items per bucket).
The following picture shows graphically the improvement obtained by the
proposed change. We used the `onnx_test_runner` command.
#### Before
```
$> time ./onnx_test_runner -v ~/folder_with_model
result:
Models: 1
Total test cases: 0
Succeeded: 0
Not implemented: 0
Failed: 0
Stats by Operator type:
Not implemented(0):
Failed:
Failed Test Cases:
real 0m55.695s
user 0m52.919s
sys 0m0.760s
```
#### After
```
$> time ./onnx_test_runner -v ~/folder_with_model
result:
Models: 1
Total test cases: 0
Succeeded: 0
Not implemented: 0
Failed: 0
Stats by Operator type:
Not implemented(0):
Failed:
Failed Test Cases:
real 0m17.152s
user 0m14.318s
sys 0m0.619s
```
