GlobalDP Tutorial: Solving serialization issue with ray backend

[scngupta-dsp]

Summary of Changes:

1. Modified requirements_global_dp.txt and reverted key dependencies to their original versions (942fccc)

• torch: 2.2.0 to 1.12.1
• torchaudio: Removed
• torchvision: 0.17.0 to 0.13.1
• opacus: 1.5.1 to 1.2.0

2. Defined global_model_tools as an aggregator private attribute via a callable
3. Removed initialization of global_model_tools from FederatedFlow

Validation:

Workflow_Interface_Mnist_Implementation_2.py is executed for single_process as well as ray backends. No errors / exceptions observed