Set eval_sample_packing to false in mistral config.yaml (#1003)

Without eval_sampling_packing set to false, ValueError occurs with eval dataset split is too small for sample_packing.

examples/mistral/config.yml

@@ -17,6 +17,7 @@ output_dir: ./out
 sequence_len: 8192
 sample_packing: true
 pad_to_sequence_len: true
+eval_sample_packing: false
 
 wandb_project:
 wandb_entity: