Add test coverage for GradientAccumulation callback

[Copilot]

Description

Addresses review feedback from PR #225 requesting test coverage for the GradientAccumulation callback class introduced in commit 8741d7f.

Changes

• New test file: tests/test_utils/test_model_utils.py with 16 tests covering:
  • lerp() helper function: basic interpolation, fractional steps, negative values, reverse ranges
  • GradientAccumulation callback:
    • Initialization with/without warmup configuration
    • Setup calculations for single/multi-GPU (world_size scaling)
    • Fractional accumulation rounding and minimum bounds
    • Warmup interpolation (gradual increase from 1 to max_accumulation)
    • Post-warmup behavior (constant max_accumulation)
    • Integration with trainer's accumulate_grad_batches setting
    • Full training cycle simulation

Example Test Coverage

# Tests warmup interpolation behavior
callback.current_batch = 75  # 25% through warmup
callback.on_train_batch_start(mock_trainer, mock_pl_module)
# Verifies accumulation interpolates: lerp(1, 4, 75, 300) → 2
assert mock_trainer.accumulate_grad_batches == 2

Type of Change

• New feature (non-breaking change which adds functionality)

Checklist:

• The code follows the Python style guide.
• Code and files are well organized.
• All tests pass.
• New code is covered by tests.
• The pull request is directed to the corresponding topic branch.
• [Optional] We would be very happy if gitmoji 🧑‍💻 could be used to assist the commit message 💬!

Licensing:

By submitting this pull request, I confirm that:

• My contribution is made under the MIT License.
• I have not included any code from questionable or non-compliant sources (GPL, AGPL, ... etc).
• I understand that all contributions to this repository must comply with the MIT License, and I promise that my contributions do not violate this license.
• I have not used any code or content from sources that conflict with the MIT License or are otherwise legally questionable.

Additional Information

All 16 tests pass. CodeQL security scan: 0 alerts.

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