consolidate the tokenizer (#14327)

This change adds a new x/tokenizer package which includes:
  * New BPE and SentencePiece tokenizers
  * Removing the dependency on the imagegen tokenizers
  * Fixes to multibyte decoding in the pipeline
  * Various correctness and benchmark tests

Not included in this PR is the WordPiece tokenizer for BERT models which will be
added when we add embedding models. The imagegen tokenizers will also be removed in
a follow-up PR.

x/tokenizer/tokenizer_correctness_test.go

@@ -0,0 +1,137 @@
+//go:build mlx
+
+package tokenizer
+
+import (
+	"runtime"
+	"strings"
+	"testing"
+)
+
+func equalIDs(a, b []int32) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i := range a {
+		if a[i] != b[i] {
+			return false
+		}
+	}
+	return true
+}
+
+func TestEncodeRoundtripMiniLlama(t *testing.T) {
+	tok := benchmarkLoadMiniLlama(t)
+
+	inputs := []string{
+		"",
+		"hello",
+		"hello world",
+		" hello  world ",
+		"don't we'll they're",
+		"1234567890",
+		"こんにちは世界",
+		"Hello 世界",
+		"func main() {}",
+		"<|begin_of_text|>system\nYou are concise.<|end_of_text|>",
+		strings.Repeat("The quick brown fox jumps over the lazy dog. ", 32),
+	}
+
+	for _, input := range inputs {
+		ids := tok.Encode(input, false)
+		got := tok.Decode(ids)
+		if got != input {
+			t.Fatalf("roundtrip mismatch for %q: got %q", input, got)
+		}
+	}
+}
+
+func TestSplitBySpecialTokensGreedyLongest(t *testing.T) {
+	data := []byte(`{
+		"model": {
+			"type": "BPE",
+			"vocab": {"a": 0, "b": 1},
+			"merges": []
+		},
+		"added_tokens": [
+			{"id": 2, "content": "<tag>", "special": true},
+			{"id": 3, "content": "<tag>x", "special": true}
+		]
+	}`)
+
+	tok, err := LoadFromBytes(data)
+	if err != nil {
+		t.Fatalf("failed to load tokenizer: %v", err)
+	}
+
+	input := "a<tag>xb"
+	want := []string{"a", "<tag>x", "b"}
+
+	got := tok.splitBySpecialTokens(input)
+	if len(got) != len(want) {
+		t.Fatalf("split length mismatch: got %v want %v", got, want)
+	}
+	for i := range want {
+		if got[i] != want[i] {
+			t.Fatalf("split mismatch at %d: got %v want %v", i, got, want)
+		}
+	}
+}
+
+func TestSplitBySpecialTokensFallbackWithoutCache(t *testing.T) {
+	data := []byte(`{
+		"model": {
+			"type": "BPE",
+			"vocab": {"a": 0, "b": 1},
+			"merges": []
+		},
+		"added_tokens": [
+			{"id": 2, "content": "<tag>", "special": true},
+			{"id": 3, "content": "<tag>x", "special": true}
+		]
+	}`)
+
+	tok, err := LoadFromBytes(data)
+	if err != nil {
+		t.Fatalf("failed to load tokenizer: %v", err)
+	}
+
+	input := "a<tag>xb"
+	want := []string{"a", "<tag>x", "b"}
+
+	// Simulate construction outside loader path where cache is not set.
+	tok.sortedSpecialTokens = nil
+
+	got := tok.splitBySpecialTokens(input)
+	if len(got) != len(want) {
+		t.Fatalf("split length mismatch: got %v want %v", got, want)
+	}
+	for i := range want {
+		if got[i] != want[i] {
+			t.Fatalf("split mismatch at %d: got %v want %v", i, got, want)
+		}
+	}
+}
+
+func TestEncodeDeterministicAcrossGOMAXPROCS(t *testing.T) {
+	tok := benchmarkLoadMiniLlama(t)
+
+	input := strings.Repeat("The quick brown fox jumps over the lazy dog. ", 640)
+
+	prev := runtime.GOMAXPROCS(0)
+	defer runtime.GOMAXPROCS(prev)
+
+	runtime.GOMAXPROCS(1)
+	seq := tok.Encode(input, false)
+
+	if prev < 2 {
+		runtime.GOMAXPROCS(2)
+	} else {
+		runtime.GOMAXPROCS(prev)
+	}
+	par := tok.Encode(input, false)
+
+	if !equalIDs(seq, par) {
+		t.Fatalf("encode mismatch between sequential and parallel paths: seq=%d par=%d", len(seq), len(par))
+	}
+}