Use build tags to generate accelerated binaries for CUDA and ROCm on Linux.

The build tags rocm or cuda must be specified to both go generate and go build.
ROCm builds should have both ROCM_PATH set (and the ROCM SDK present) as well
as CLBlast installed (for GGML) and CLBlast_DIR set in the environment to the
CLBlast cmake directory (likely /usr/lib/cmake/CLBlast). Build tags are also
used to switch VRAM detection between cuda and rocm implementations, using
added "accelerator_foo.go" files which contain architecture specific functions
and variables. accelerator_none is used when no tags are set, and a helper
function addRunner will ignore it if it is the chosen accelerator. Fix go
generate commands, thanks @deadmeu for testing.

llm/accelerator_none.go

@@ -0,0 +1,21 @@
+//go:build !rocm && !cuda
+
+package llm
+
+import (
+	"errors"
+)
+
+var (
+	errNoAccel = errors.New("no accelerator support in this binary")
+)
+
+// acceleratedRunner returns the runner for this accelerator given the provided buildPath string.
+func acceleratedRunner(buildPath string) []ModelRunner {
+	return make([]ModelRunner, 0, 1)
+}
+
+// CheckVRAM is a stub with no accelerator.
+func CheckVRAM() (int64, error) {
+	return 0, errNoGPU
+}