Release update 20250331 (#2460) [skip ci]

* make torch 2.6.0 the default image

* fix tests against upstream main

* fix attribute access

* use fixture dataset

* fix dataset load

* correct the fixtures + tests

* more fixtures

* add accidentally removed shakespeare fixture

* fix conversion from unittest to pytest class

* nightly main ci caches

* build 12.6.3 cuda base image

* override for fix from huggingface/transformers#37162

* address PR feedback

.github/workflows/base.yml

@@ -40,6 +40,12 @@ jobs:
             python_version: "3.11"
             pytorch: 2.6.0
             torch_cuda_arch_list: "7.0 7.5 8.0 8.6 8.7 8.9 9.0+PTX"
+          - cuda: "126"
+            cuda_version: 12.6.3
+            cudnn_version: ""
+            python_version: "3.11"
+            pytorch: 2.6.0
+            torch_cuda_arch_list: "7.0 7.5 8.0 8.6 8.7 8.9 9.0+PTX"
           - cuda: "128"
             cuda_version: 12.8.1
             cudnn_version: ""