refactor separable solver

[Mohitbalwani26]

References to other Issues or PRs

SEE #18348

Brief description of what is fixed or changed

Refactored separable hint.

Other comments

ode_separable method is still in ode.py as separable_reduced hint uses it.

Release Notes

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#### References to other Issues or PRs
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SEE #18348 

#### Brief description of what is fixed or changed
Refactored separable hint.

#### Other comments
 `ode_separable` method is still in `ode.py` as `separable_reduced` hint uses it.

#### Release Notes

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[oscarbenjamin]

Looks good, Thanks!