Add layers_per_ipu

Add layers_per_ipu to wav2vec2 ipu_config.json, so that the mapping of layers to IPUs can be configured

ipu_config.json

@@ -11,5 +11,6 @@
     "gradient_accumulation_steps": 16,
     "executable_cache_dir": "./exe_cache",
     "ipus_per_replica": 8,
+    "layers_per_ipu": [2, 1, 4, 3, 3, 3, 3, 0],
     "matmul_proportion": [0.1, 0.1, 0.1, 0.3, 0.3, 0.3, 0.3, 0.1]
 }