Make step activation inclusive at the threshold

[Pablosinyores]

The step activation documents its boundary as inclusive:

step(x) = 0  if x < threshold
          1  if x >= threshold

Both nn.step and the nn.Step module render this x >= threshold form. The implementation, however, used a strict comparison:

return mx.where(x > threshold, 1, 0)

so an input exactly equal to the threshold returned 0 instead of 1. With the default threshold=0 this means step(0.0) evaluates to 0, which contradicts the documented behavior.

Switched to x >= threshold so the value at the boundary is included, matching the docstring. Added a test_step case covering the boundary for both the functional and module forms.

[angeloskath]

Thanks!