Unify Backend API

[kprokofi]

Standardize the public API exposed by all training backends to provide a consistent developer experience regardless of the underlying framework.

Goals

• Define a common backend interface
• Standardize training, validation, export, and inference APIs
• Harmonize configuration options across backends
• Expose common capabilities while gracefully handling backend-specific features
• Reduce backend-specific conditional logic throughout the codebase

Acceptance Criteria

• All backends implement the same public interface
• Common operations behave consistently across backends
• Backend-specific features are clearly documented and isolated
• Documentation reflects the unified API

[kprokofi]

This PR resolved half of the problems: #6874