Convert activation functions to numpower

[SkibidiProduction]

Activation implementations

• Swapped out custom Tensor code for NumPower APIs across all functions: ReLU, LeakyReLU, ELU, GELU, HardSigmoid, SiLU, Tanh, Sigmoid, Softmax, Softplus, Softsign, ThresholdedReLU, etc.

• Updated derivative methods to use numpower’s derivative helpers.

Tests

• Refactored unit tests to assert against numpower outputs.

• Adjusted tolerances and assertions to match numpower’s numeric behavior.

Documentation

• Added/updated images under docs/images/activation-functions/ to illustrate each activation curve and its derivative using the new implementations.

• Cleaned up corresponding markdown to reference the updated diagrams.

Code cleanup

• Aligned naming conventions and method signatures with numpower’s API.

• Minor style fixes (whitespace, imports, visibility).