Testing cleanup: remove redundant tests, reduce memory usage, replace materialize_meta_tensors

[jlamypoirier]

✨ Description

Extracted from #289 to reduce its size:

• Remove tests in test_mtp and test_ssm that are made redundant by Run tests for all models #289
• Reduce resource usage of test_preference_logps and test_dropless_mlp.
• Replace materialize_meta_tensors with the safer get_stage.

🔍 Type of change

Select all that apply:

• 🐛 Bug fix (non-breaking change that addresses a specific issue)
• 🚀 New feature (non-breaking change that adds functionality)
• ⚠️ Breaking change (a change that could affect existing functionality)
• 📈 Performance improvement/optimization (improves speed, memory usage, or efficiency)
• 🛠️ Code refactor (non-functional changes that improve code readability, structure, etc.)
• 📦 Dependency bump (updates dependencies, including Dockerfile or package changes)
• 📝 Documentation change (updates documentation, including new content or typo fixes)
• 🔧 Infrastructure/Build change (affects build process, CI/CD, or dependencies)