remove internal usage of within_gradient

`within_gradient` currently is not supported by Enzyme and Mooncake. Either remove internal usage (e.g. in softmax second order derivatives) or extend support to all AD frameworks.

Start by locating internal uses of `within_gradient`, especially in softmax second-order derivatives, and review how Enzyme and Mooncake handle the operation. Determine whether the scope is removing those uses or extending support across all AD frameworks; done means the chosen approach works with both named frameworks and preserves the affected derivative behavior.

Written by the indexing model from the issue text.