Update to how gradientCheckpointing works.

WORKSPACE

@@ -5,9 +5,9 @@ load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 
 git_repository(
     name = "ccv",
-    commit = "e7c76392ec4ad529797484c6759fd2f8da974d99",
+    commit = "20d998dc3c7008060df6fdfa17e932c9dae93d31",
     remote = "https://github.com/liuliu/ccv.git",
-    shallow_since = "1728857981 -0400",
+    shallow_since = "1729535387 -0400",
 )
 
 load("@ccv//config:ccv.bzl", "ccv_deps", "ccv_setting")

deps.bzl

@@ -17,8 +17,8 @@ def s4nnc_deps():
         git_repository,
         name = "ccv",
         remote = "https://github.com/liuliu/ccv.git",
-        commit = "e7c76392ec4ad529797484c6759fd2f8da974d99",
-        shallow_since = "1728857981 -0400",
+        commit = "20d998dc3c7008060df6fdfa17e932c9dae93d31",
+        shallow_since = "1729535387 -0400",
     )
 
     _maybe(

nnc/Model.swift

@@ -205,12 +205,23 @@ public class Model {
    * Whether to enable gradient checkpointing for this model. Once it is enabled, we will re-run
    * the model forward pass again during backward pass. This is effective at reducing memory usage.
    */
-  public var gradientCheckpointing: Bool {
+  public var gradientCheckpointing: Bool? {
     get {
-      ccv_cnnp_model_gradient_checkpointing(cModel) != 0
+      let value = ccv_cnnp_model_gradient_checkpointing(cModel)
+      if value == 1 {
+        return true
+      } else if value == -1 {
+        return false
+      } else {
+        return nil
+      }
     }
     set {
-      ccv_cnnp_model_set_gradient_checkpointing(cModel, newValue ? 1 : 0)
+      if let newValue = newValue {
+        ccv_cnnp_model_set_gradient_checkpointing(cModel, newValue ? 1 : -1)
+      } else {
+        ccv_cnnp_model_set_gradient_checkpointing(cModel, 0)
+      }
     }
   }