mlxrunner: introduce ForwardBatch for model forward pass

Replace the raw *mlx.Array token input with a ForwardBatch struct
that carries InputIDs alongside sequence metadata (SeqIDs, SeqLens).
InputIDs remain [1, N] shaped — all model code is unchanged beyond
the signature.

x/mlxrunner/batch/batch.go

@@ -0,0 +1,32 @@
+package batch
+
+import "github.com/ollama/ollama/x/mlxrunner/mlx"
+
+// ForwardBatch carries per-step metadata through the model forward pass.
+// InputIDs shape is [1, N] where N = sum(SeqLens). SeqLens indicates how
+// many tokens belong to each sequence.
+type ForwardBatch struct {
+	// InputIDs holds token IDs across all sequences. Shape: [1, N].
+	InputIDs *mlx.Array
+
+	// SeqIDs uniquely identifies each sequence in the batch.
+	SeqIDs []int
+
+	// SeqLens is the number of new tokens per sequence in this step.
+	// For decode batching every entry is 1. For prefill it may vary.
+	SeqLens []int
+}
+
+// TotalLen returns the total number of tokens across all sequences.
+func (b *ForwardBatch) TotalLen() int {
+	n := 0
+	for _, l := range b.SeqLens {
+		n += l
+	}
+	return n
+}
+
+// NumSeqs returns the number of sequences in the batch.
+func (b *ForwardBatch) NumSeqs() int {
+	return len(b.SeqIDs)
+}