[onert] Unify interfaces of TrainableBackendContext

[ragmani]

What

Let's unify interfaces(genTensors, genTrainingTensors, and genKernels) of TrainableBackendContext.

Why

Currently those three functions(genTensors, genTrainingTensors, and genKernels) are called sequentially because the creation of tensors is the prerequisite for generating kernels in most backends and generating kernels has no prerequisite related to generating tensors.
However, we are going to introduce a feature in which generating kernels is the prerequisite for generating tensors in the train backend. That means generating tensors and generating kernels are interdependent on each other. Therefore, the calling priority of those three functions cannot be determined outside the train backend. We need to pass the determination to each training backend instead of onert core.

draft : #13629

• [onert] Unify interface for generating tensors #13631
• [onert] Unify generating training tensors and kernels #13656

[ragmani]

Done.