✨[Feature] Add `GetITensor` constant management system to Dynamo

[gs-olive]

Context

In the TorchScript path of the codebase, tensors are accessed via the ITensorOrFreeze function, as below:

TensorRT/core/conversion/var/Var.cpp

Line 89 in ea5a289

nvinfer1::ITensor* Var::ITensorOrFreeze(ConversionCtx* ctx) {

This function serves to freeze any input constants before returning them. Such a utility would be very useful in Dynamo, to handle cases where converters cannot process Numpy array or Torch Tensor inputs (like aten.cat, for example).

Converters could use this function when a certain input is required to be an ITensor, and omit it when this is not required.