pnnx fuse more rmsnorm pattern

[nihui]

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[Copilot]

Pull Request Overview

This PR adds support for an additional RMSNorm pattern variant and improves variable naming for better code organization.

• Adds a new fusion pass (fuse_rmsnorm_pass_2) to match an alternative algebraic form of RMSNorm computation
• Renames variables in the fuse_rmsnorm function to use semantic prefixes (a for passes with gamma/weight parameters, g for passes without gamma)

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[github-actions]

The binary size change of libncnn.so (bytes)

architecture	base size	pr size	difference
x86_64	15244144	15244144	0 😘
armhf	6225824	6225824	0 😘
aarch64	9526976	9526976	0 😘