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(tf.constant(["1 1:1 2:2", "2 1:2 2:3"]), 2, dtype=tf.float32, label_dtype=tf.int64)==(tf.constant([1, 2], dtype=tf.int64), tf.constant([[0, 0], [0, 1], [1, 0], [1, 1]], dtype=tf.int64), tf.constant([1.0, 2.0, 2.0, 3.0], dtype=tf.float32), tf.constant([2, 2], dtype=tf.int64))
  assert decode_libsvm(tf.constant(["1 1:1 2:2"]), 2, dtype=tf.float64, label_dtype=tf.float32)==(tf.constant([1.0], dtype=tf.float32), tf.constant([[0, 0], [0, 1]], dtype=tf.int64), tf.constant([1.0, 2.0], dtype=tf.float64), tf.constant([1, 2], dtype=tf.int64))
  assert decode_libsvm(tf.constant([""]), 2, dtype=tf.int32, label_dtype=tf.int64)==(tf.constant([0], dtype=tf.int64), tf.constant([], dtype=tf.int64), tf.constant([], dtype=tf.int32), tf.constant([1, 2], dtype=tf.int64))
output: def decode_libsvm(input, num_features, dtype=tf.float32, label_dtype=tf.int64, name=None):
      with tf.name_scope(name, "decode_libsvm", [input, num_features, dtype, label_dtype]):
          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:]:
                  if part:
                      idx, val = tf.strings.split(part, ':')
                      indices.append(tf.strings.to_number(idx, out_type=tf.int64) - 1)
                      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),
              infer_shape=False, name="map_parse_line"
          )
  
          labels = tf.stack(labels)
          indices = tf.ragged.stack(indices).flat_values
          values = tf.ragged.stack(values).flat_values
          feature_shape = tf.constant([tf.shape(input)[0], num_features], dtype=tf.int64)
  
          return labels, indices, values, feature_shape