Support method & signature name selection during Inference

[asamadiya]

Is your feature request related to a problem? Please describe.
Support loading models with multiple signatures and selection of a particular signature during model inference. This is supported by standard inference servers like TF Serving and libraries like libtensorflow & libtorch.

Our models have multiple methods (PyTorch) or signatures (TensorFlow) and based on the operation we perform, we have to choose the correct signature at inference time.

Describe the solution you'd like

1. Support loading multiple signatures at model loading time for Tensorflow, Torch and any backend as long as it supports it.
2. During inference time, fix a InferenceRequest.parameter key "SIGNATURE_NAME", when provided, will allow the backend to run the model on that signature. In PyTorch that would be "METHOD_NAME".

Describe alternatives you've considered
Instead of using Triton, using the model framework libraries directly such as libtensorflow and libtorch.

Additional context
Simple example is a model can do a forward pass, a forward pass with additional ranking and produce new feature tensor and these "signatures" can share most of the model graph so having them in one model is our desire.