Asserts and boolean checks

• tf.assert_negative
• tf.assert_positive
• tf.assert_proper_iterable
• tf.assert_non_negative
• tf.assert_non_positive
• tf.assert_equal
• tf.assert_integer
• tf.assert_less
• tf.assert_less_equal
• tf.assert_greater
• tf.assert_greater_equal
• tf.assert_rank
• tf.assert_rank_at_least
• tf.assert_type
• tf.is_non_decreasing
• tf.is_numeric_tensor
• tf.is_strictly_increasing