model: implement bert in ollama engine (#9080)

* fix truncate

* s/SentencePieceModel/SentencePiece/

* bert

* wordpiece

* refactor pooling

* more tokenizers

* normalize embeddings

ml/nn/pooling/pooling.go

@@ -0,0 +1,36 @@
+package pooling
+
+import (
+	"github.com/ollama/ollama/ml"
+)
+
+type Type uint32
+
+const (
+	TypeNone Type = iota
+	TypeMean
+	TypeCLS
+	TypeLast
+	TypeRank
+
+	TypeUnknown     = 0xFFFFFFFE
+	TypeUnspecified = 0xFFFFFFFF
+)
+
+func Pooling(ctx ml.Context, hiddenStates ml.Tensor, poolingType Type) ml.Tensor {
+	switch poolingType {
+	case TypeNone:
+		return hiddenStates
+	case TypeMean:
+		hiddenStates = hiddenStates.Permute(ctx, 1, 0, 2, 3).Contiguous(ctx).Mean(ctx)
+		return hiddenStates.Permute(ctx, 1, 0, 2, 3).Contiguous(ctx)
+	case TypeCLS:
+		return hiddenStates.View(ctx, 0, hiddenStates.Dim(0))
+	case TypeLast:
+		panic("not implemented")
+	case TypeRank:
+		panic("not implemented")
+	default:
+		panic("not implemented")
+	}
+}