llm: Allow overriding flash attention setting

As we automatically enable flash attention for more models, there
are likely some cases where we get it wrong. This allows setting
OLLAMA_FLASH_ATTENTION=0 to disable it, even for models that usually
have flash attention.

llm/server.go

@@ -196,14 +196,10 @@ func NewLlamaServer(gpus discover.GpuInfoList, modelPath string, f *ggml.GGML, a
 		loadRequest.ProjectorPath = projectors[0]
 	}
 
+	fa := envconfig.FlashAttention(f.FlashAttention())
+
 	// This will disable flash attention unless all GPUs on the system support it, even if we end up selecting a subset
 	// that can handle it.
-	fa := envconfig.FlashAttention()
-	if f.FlashAttention() {
-		slog.Info("model wants flash attention")
-		fa = true
-	}
-
 	if fa && !gpus.FlashAttentionSupported() {
 		slog.Warn("flash attention enabled but not supported by gpu")
 		fa = false