Add Gemma

[yisi-wang-slalom]

Description of changes:

We are excited to help adding support for Gemma model architectured by Google.

A few highlights, mainly referencing: huggingface/transformers#29402

• Using gelu_new activation function, we've validated it against the PyTorch implementation of gelu_pytorch_tanh to ensure same result, and recommend implementing gelu_pytorch_tanh when possible.
• Add Layernorm (w+1) of RMS Layernorm for gemma
• Include additional normalization in embedding (multiplies the embeddings by sqrt(hidden_dim)) for gemma

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