launcher: correctly get the head dimension for VLMs (#3116)

* launcher: correctly get the head dimension for VLMs

For most (?) VLMs, the head dimension is in the `text_config`
configuration section. However, since we only queried the top-level
`head_dim` (which typically doesn't exist in VLMs), we would never use
flashinfer. This change adds a method that gets the head dimension from
the top-level `Config` struct or `text_config` when that fails.

* fix: bump org name in gemma3 test

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Co-authored-by: drbh <david.richard.holtz@gmail.com>