Fix calc_stats dataset mutation causing incorrect diff_std computation

[RajdeepKushwaha5]

Describe your changes

The calc_stats function in mllam_data_prep/ops/statistics.py mutates the ds variable inside the loop over statistics_config.ops. When a diff_* operation is encountered, the original dataset is overwritten:

ds = ds[vars_to_keep].diff(dim=splitting_dim)

This means any subsequent diff_* operation in the same loop applies .diff() on an already-diffed dataset instead of the original.

With the default config shipped in all example YAML files (ops: [mean, std, diff_mean, diff_std]):

1. mean → ds.mean(...) ✅
2. std → ds.std(...) ✅
3. diff_mean → mutates ds to ds.diff(...), then mean(...) ✅ (but corrupts ds)
4. diff_std → applies .diff() again on already-diffed data → computes std(diff(diff(ds))) ❌

Expected: diff_std should compute std(diff(ds)).
Actual: diff_std computes std(diff(diff(ds))).

Fix: Use a per-iteration local variable ds_op instead of mutating ds, so each iteration always starts from the original dataset.

No dependencies required for this change.

Issue Link

Closes #102

Type of change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📖 Documentation (Addition or improvements to documentation)

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• I have added a line to the CHANGELOG describing this change, in a section
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[Copilot]

Pull request overview

Fixes a logic error in calc_stats where encountering a diff_* op would overwrite the dataset used for subsequent ops, leading to incorrect results (e.g., diff_std computing a double diff when diff_mean appears earlier in the same ops list). This aligns behavior with the default YAML configs that compute both base and diff-based statistics.

Changes:

• Avoid reassigning/mutating the loop dataset by introducing a per-iteration ds_op variable in calc_stats.
• Add regression tests covering multiple diff_* ops in a single ops list (verifies diff_std uses a single diff of the original dataset).
• Document the fix in the unreleased CHANGELOG (links to issue #102).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
mllam_data_prep/ops/statistics.py	Ensures each statistic op runs against the intended (original) dataset by using a per-iteration dataset variable (ds_op).
tests/test_calc_stats_fix.py	Adds regression coverage to prevent reintroducing the incorrect diff_std behavior.
CHANGELOG.md	Records the bugfix under “Fixes” in the unreleased section.

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