Support multiple variants for a given llm lib type

In some cases we may want multiple variants for a given GPU type or CPU.
This adds logic to have an optional Variant which we can use to select
an optimal library, but also allows us to try multiple variants in case
some fail to load.

This can be useful for scenarios such as ROCm v5 vs v6 incompatibility
or potentially CPU features.

gpu/types.go

@@ -11,5 +11,8 @@ type GpuInfo struct {
 	memInfo
 	Library string `json:"library,omitempty"`
 
+	// Optional variant to select (e.g. versions, cpu feature flags)
+	Variant string `json:"variant,omitempty"`
+
 	// TODO add other useful attributes about the card here for discovery information
 }