tensorflow creating mask of varied lengths

This can be achieved using a variety of TensorFlow transformations:

# Make a 4 x 8 matrix where each row contains the length repeated 8 times.
lengths = [4, 3, 5, 2]
lengths_transposed = tf.expand_dims(lengths, 1)

# Make a 4 x 8 matrix where each row contains [0, 1, ..., 7]
range = tf.range(0, 8, 1)
range_row = tf.expand_dims(range, 0)

# Use the logical operations to create a mask
mask = tf.less(range_row, lengths_transposed)

# Use the select operation to select between 1 or 0 for each value.
result = tf.select(mask, tf.ones([4, 8]), tf.zeros([4, 8]))

This can now be achieved by tf.sequence_mask. More details here.