Add kokoro_duration_t512 for v1.6.1 enumerated-token duration models

coreml/kokoro_duration_t512.mlpackage/Data/com.apple.CoreML/model.mlmodel

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+version https://git-lfs.github.com/spec/v1
+oid sha256:a56ff11bf527ae9919e824b36bc85bab868538b13dd662a67b3f866e5d72d37f
+size 22002668

coreml/kokoro_duration_t512.mlpackage/Data/com.apple.CoreML/weights/weight.bin

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+version https://git-lfs.github.com/spec/v1
+oid sha256:4abcd679c802f3ab73f5548f1f6b954997306d8801114773a33b108ec5cf609b
+size 39115520

coreml/kokoro_duration_t512.mlpackage/Manifest.json

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+{
+    "fileFormatVersion": "1.0.0",
+    "itemInfoEntries": {
+        "555C4B1F-EAEE-4177-8BA9-3CEF8ED9A642": {
+            "author": "com.apple.CoreML",
+            "description": "CoreML Model Specification",
+            "name": "model.mlmodel",
+            "path": "com.apple.CoreML/model.mlmodel"
+        },
+        "C33A955F-0880-498E-83DA-74C9F1B42568": {
+            "author": "com.apple.CoreML",
+            "description": "CoreML Model Weights",
+            "name": "weights",
+            "path": "com.apple.CoreML/weights"
+        }
+    },
+    "rootModelIdentifier": "555C4B1F-EAEE-4177-8BA9-3CEF8ED9A642"
+}