Move fit computation code to dedicated new file (#10174)

* Move fit computation code to dedicated new file

Part of #10089

* .

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: max-sixty