Allow reading either 4:2:0 chroma format.

PiperOrigin-RevId: 555969699
Change-Id: I44f05c10877b6a93825280e9deea5f87f6546dad

Author: Johannes Ballé

tensorflow_compression/cc/kernels/y4m_dataset_kernels.cc

@@ -349,6 +349,8 @@ class Y4MDatasetOp : public DatasetOpKernel {
             case 'C':
               if (absl::ConsumePrefix(&header, "420jpeg")) {
                 chroma_format = ChromaFormat::I420;
+              } else if (absl::ConsumePrefix(&header, "420")) {
+                chroma_format = ChromaFormat::I420;
               } else if (absl::ConsumePrefix(&header, "444")) {
                 chroma_format = ChromaFormat::I444;
               } else {