perf: build graph for next batch async to keep GPU busy (#11863)

* perf: build graph for next batch in parallel to keep GPU busy

This refactors the main run loop of the ollama runner to perform the main GPU
intensive tasks (Compute+Floats) in a go routine so we can prepare the next
batch in parallel to reduce the amount of time the GPU stalls waiting for the
next batch of work.

* tests: tune integration tests for ollama engine

This tunes the integration tests to focus more on models supported
by the new engine.

ml/backend.go

@@ -372,6 +372,7 @@ type Context interface {
 
 	Forward(...Tensor) Context
 	Compute(...Tensor)
+	ComputeWithNotify(func(), ...Tensor) // notify callback once compute has begun
 
 	// Reserve is analogous to Compute but rather than executing a
 	// graph, simply preallocates memory. Typically called with a
@@ -401,6 +402,8 @@ type Tensor interface {
 	Bytes() []byte
 	Floats() []float32
 
+	SetValueFromIntSlice(s []int32)
+
 	Neg(ctx Context) Tensor
 	Add(ctx Context, t2 Tensor) Tensor
 	Sub(ctx Context, t2 Tensor) Tensor