ty: un-ignore call-non-callable rule - #15582

Describe your change

Part of #15187. call-non-callable had a single diagnostic left, in neural_network/back_propagation_neural_network.py: DenseLayer.activation started out as None and was only replaced with sigmoid later in initializer(), so ty saw a call on a possibly None attribute. The default is now resolved in __init__ and the parameter is annotated, which clears the diagnostic, so the rule is removed from the [tool.ty] ignore list. The input layer never calls its activation, so behaviour is unchanged.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Checklist

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
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