Fix kv_channels configuration for Gemma2 27b

[ananthsub]

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What does this PR do ?

https://huggingface.co/google/gemma-2-27b/blob/main/config.json#L12
kv_channels for the megatron core config corresponding to head_dim in the HF config should be 128 for Gemma2 27B, not 256 as the other Gemma models have

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