input: Write a function to decode LibSVM input to tensors using TensorFlow ops. The function should be named `decode_libsvm` and take the following arguments: `input` (a `Tensor` of type `string`), `num_features` (an `int` that is `>= 1`), `dtype` (an optional `tf.DType` from: `tf.float32, tf.float64, tf.int32, tf.int64`. Defaults to `tf.float32`), `label_dtype` (an optional `tf.DType` from: `tf.float32, tf.float64, tf.int32, tf.int64`. Defaults to `tf.int64`), and `name` (a name for the operation, optional). The function should return a tuple of `Tensor` objects `(label, feature_indices, feature_values, feature_shape)`, where `label` is a `Tensor` of type `label_dtype`, `feature_indices` is a `Tensor` of type `int64`, `feature_values` is a `Tensor` of type `dtype`, and `feature_shape` is a `Tensor` of type `int64`.
  
  assert decode_libsvm(input=tf.constant(["0 1:1 3:1", "1 2:1 4:1"]), num_features=5, dtype=tf.float32, label_dtype=tf.int32) == (tf.constant([0, 1], dtype=tf.int32), tf.constant([[0, 1], [0, 3], [1, 2], [1, 4]], dtype=tf.int64), tf.constant([1, 1, 1, 1], dtype=tf.float32), tf.constant([2], dtype=tf.int64))
  assert decode_libsvm(input=tf.constant([]), num_features=3, dtype=tf.float32, label_dtype=tf.int64) == (tf.constant([], dtype=tf.int64), tf.constant([], dtype=tf.int64), tf.constant([], dtype=tf.float32), tf.constant([0], dtype=tf.int64))
  assert decode_libsvm(input=tf.constant(["0 1:3", "1 2:4"]), num_features=3, dtype=tf.int32, label_dtype=tf.int32) == (tf.constant([0, 1], dtype=tf.int32), tf.constant([[0, 1], [1, 2]], dtype=tf.int64), tf.constant([3, 4], dtype=tf.int32), tf.constant([2], dtype=tf.int64))
  assert decode_libsvm(input=tf.constant(["0 1:1", "1 2:1"]), num_features=2, dtype=tf.float64, label_dtype=tf.float64) == (tf.constant([0, 1], dtype=tf.float64), tf.constant([[0, 1], [1, 2]], dtype=tf.int64), tf.constant([1, 1], dtype=tf.float64), tf.constant([2], dtype=tf.int64))
  assert decode_libsvm(input=tf.constant([]), num_features=1, dtype=tf.float64, label_dtype=tf.float64) == (tf.constant([], dtype=tf.float64), tf.constant([], dtype=tf.int64), tf.constant([], dtype=tf.float64), tf.constant([0], dtype=tf.int64))
output: def decode_libsvm(input, num_features, dtype=tf.float32, label_dtype=tf.int64, name=None):
      def parse_line(line):
          parts = tf.strings.split(line, ' ')
          label = tf.strings.to_number(parts[0], out_type=label_dtype)
          indices = []
          values = []
          for part in parts[1:]:
              idx, val = tf.strings.split(part, ':')
              indices.append([len(indices), tf.strings.to_number(idx, out_type=tf.int64)])
              values.append(tf.strings.to_number(val, out_type=dtype))
          return label, indices, values
  
      labels, indices, values = tf.map_fn(
          parse_line,
          input,
          dtype=(label_dtype, tf.int64, dtype),
          name=name
      )
  
      labels = tf.stack(labels)
      indices = tf.reshape(tf.stack(indices), [-1, 2])
      values = tf.reshape(tf.stack(values), [-1])
      feature_shape = tf.shape(input, out_type=tf.int64)
  
      return labels, indices, values, feature_shape