Add requirements.txt

[FedericoBruzzone]

This pull request updates the documentation to clarify the dependencies required for running the model conversion and compilation script. The main change is an improvement to the instructions in the README.md regarding the Python environment and required dependencies.

Documentation improvements:

• Clarified that the venv Python environment must have all necessary dependencies installed, as specified in requirements.txt, for the iree.sh script to work properly.

[bernborgess]

The issue with setting the requirements.txt file is that not all dependencies are in standard python repos. To install the required packages:

• torch-mlir

pip install --pre torch-mlir torchvision \
  --extra-index-url https://download.pytorch.org/whl/nightly/cpu \
  -f https://github.com/llvm/torch-mlir-release/releases/expanded_assets/dev-wheels

• Other deps

pip install tensorflow tqdm tf2onnx onnx