def is_sparse(tensor):
"""Returns whether a tensor is a sparse tensor.
# Arguments
tensor: A tensor instance.
# Returns
A boolean.
# Example
```python
>>> from keras import backend as K
>>> a = K.placeholder((2, 2), sparse=False)
>>> print(K.is_sparse(a))
False
>>> b = K.placeholder((2, 2), sparse=True)
>>> print(K.is_sparse(b))
True
"""
return isinstance(tensor, tf.SparseTensor)```
