Material n and k caching causes RuntimeError in torch + grad_mode

[manuelFragata]

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Bug Report

Describe the bug
Material n / k caching causes RuntimeError under torch + grad mode

When using the torch backend with gradient tracking enabled, calling .backward() more than once during optimization raises the RuntimeError saying "Trying to backward through the graph a second time...".

To Reproduce
Run the torch optimization notebooks with the most recent optiland version, and you will get the RuntimeError

Expected behavior
Optimization should proceed.

Environment

• Optiland Version: latest version

[manuelFragata]

solved in #485