[nnx] add reseed

[cgarciae]

What does this PR do?

Adds nnx.reseed to update the nested keys of a graph node.

class Model(nnx.Module):
  def __init__(self, rngs):
    self.linear = nnx.Linear(2, 3, rngs=rngs)
    self.dropout = nnx.Dropout(0.5, rngs=rngs)

  def __call__(self, x):
    return self.dropout(self.linear(x))

model = Model(nnx.Rngs(params=0, dropout=42))
x = jnp.ones((1, 2))

y1 = model(x)

# reset the ``dropout`` stream key to 42
nnx.reseed(model, dropout=42)
y2 = model(x)

assert jnp.allclose(y1, y2)