Standardize model directory structure across frameworks

[ctr-lelanchelianTT]

Problem description

The tt-forge-models repository currently uses multiple directory layouts depending on
the framework and number of tasks:

• JAX models follow: model_name / task / framework / model_code
• PyTorch models with a single task follow: model_name / framework / model_code
• PyTorch models with multiple tasks follow: model_name / task / framework / model_code

With plans to add ONNX and Paddle models, maintaining multiple layout conventions
introduces inconsistency, increases maintenance overhead, and complicates automation
and tooling.

the new structure is :

└── <model_name>
    └── <task>           
        └── <framework> 
                └── init.py
                    loader.py

eg: for alexnet model

└── <alexnet>
    └── <image_classification>           
        └── <pytorch> 
                └── init.py
                    loader.py

What's changed

All models are now standardized to a single unified directory structure

I have attached the logs for your reference:
collect_all_models.log

I have verified the changes by making sure the tests that changed are getting collected