Computes mask on sub-episodes which includes only contiguous components.
Tensor of shape [batch_size, total_steps]
corresponding to environment discounts on next time steps (i.e.
A float Tensor of shape [batch_size, total_steps] specifying mask including
only contiguous components. Each row will be of the form
[1.0] * a + [0.0] * b, where a >= 1 and b >= 0, and in which the initial
sequence of ones corresponds to a contiguous sub-episode.