Avoid reassignment in interface functions

[ValerianRey]

In backward and mtl_backward, we're defining an interface for the users. This means that we accept many different types for the parameters, and we then change their type to use our internal machinery with the required types. This is not something that mypy likes, because it leads to assignment errors. We could use the --allow-redefinition flag of mypy, but I think they're kind of right in not allowing redefinitions by default. With redefinitions, it's harder to read the code (since variable types change throughout the same function) and it makes refactors harder (because you may want to rename just the variable after it changed type for instance). We could also solve it using cast, but it's more or less the same as allowing the redefinition but just locally: it tells mypy "trust me, the type is [...]". So I think the cleanest solution is to use different variables after type has changed. Naming them can be challenging, because we don't like to have type included in the name, but at the same time, in these interfaces, we have two types for the same thing. My solution is to have a new convention to name xyz the original variable and xyz_ the one that has been changed to the right format / type.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
src/torchjd/_autojac/_backward.py	100.00% <100.00%> (ø)
src/torchjd/_autojac/_mtl_backward.py	100.00% <100.00%> (ø)

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