Put back open in colab markers (#14684)

docs/source/perplexity.mdx

@@ -12,6 +12,8 @@ specific language governing permissions and limitations under the License.
 
 # Perplexity of fixed-length models
 
+[[open-in-colab]]
+
 Perplexity (PPL) is one of the most common metrics for evaluating language models. Before diving in, we should note
 that the metric applies specifically to classical language models (sometimes called autoregressive or causal language
 models) and is not well defined for masked language models like BERT (see [summary of the models](model_summary)).