Add Python package structure to models directory

[Copilot]

The models/ and models/recursive_reasoning/ directories lacked __init__.py files, relying on implicit namespace packages. This PR adds proper package structure.

Changes

• models/__init__.py: Establishes models as a package, exports core utilities (EMA, layers, losses, sparse embedding)
• models/recursive_reasoning/__init__.py: Establishes recursive_reasoning subpackage, exports all model variants (TRM, HRM, transformers baseline, ERS_PMLL, etc.)

Implementation Details

Both files define __all__ for API documentation but avoid eager imports to prevent torch dependency loading on package import:

# models/recursive_reasoning/__init__.py
"""Recursive reasoning models for TinyRecursiveModels.

Classes are available via explicit imports:
    from models.recursive_reasoning.trm import TinyRecursiveReasoningModel_ACTV1
"""

__all__ = [
    "TinyRecursiveReasoningModel_ACTV1",
    "HierarchicalReasoningModel_ACTV1",
    # ...
]

Backward compatible with existing dynamic model loading via importlib.import_module() used in config-driven instantiation.

Original prompt

https://github.com/drqsatoshi/TinyRecursiveModels/tasks/7d925690-6100-4dad-9a4b-fa1950629350?session_id=27d34d06-1d4f-47e6-a171-0d6660c65d14

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