This warning is printed when a sparse tf.IndexedSlices object is implicitly converted to a dense tf.Tensor. This typically happens when one op (usually tf.gather()) backpropagates a sparse gradient, but the op that receives it does not have a specialized gradient function that can handle sparse gradients. As a result, TensorFlow automatically densifies the tf.IndexedSlices, which can have a devastating effect on performance if the tensor is large.
To fix this problem, you should try to ensure that the params input to tf.gather() (or the params inputs to tf.nn.embedding_lookup()) is a tf.Variable. Variables can receive the sparse updates directly, so no conversion is needed. Although tf.gather() (and tf.nn.embedding_lookup()) accept arbitrary tensors as inputs, this may lead to a more complicated backpropagation graph, resulting in implicit conversion.