Add `model_input_keys`for g2p model

See https://github.com/speechbrain/speechbrain/pull/2600 for rationale and details

hyperparams.yaml

@@ -304,6 +304,11 @@ beam_searcher_valid: !new:speechbrain.decoders.S2SRNNBeamSearcher
 
 homograph_extractor: !new:speechbrain.lobes.models.g2p.homograph.SubsequenceExtractor
 
+# keys to forward from the encoding pipeline to the model
+model_input_keys:
+- grapheme_encoded
+- word_emb
+
 model_output_keys:
 - p_seq
 - char_lens