Build one dropout keep mask per layer from a single integer seed, following an exact key-splitting rule.

Dropout randomly zeroes activations during training. A keep mask holds True for every entry that survives, and each entry is kept with probability keep_prob. The masks are fully determined by the seed and by the way keys are split, so follow this rule exactly.

1. Create the starting key with jax.random.key(seed). It is the running key.
2. For each layer in order, split the running key into two keys with jax.random.split. The first of the two replaces the running key, and the second belongs to the current layer.
3. Draw the layer's mask with jax.random.bernoulli, passing that layer key, keep_prob and the mask shape.

Return a bool JAX array of shape (num_layers, followed by the entries of shape), for example (3, 2, 4) for three layers of shape [2, 4], with the mask of layer i at index i of the first axis.