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/main.yml

@@ -25,12 +25,12 @@ jobs:
             python_version: "3.11"
             pytorch: 2.5.1
             axolotl_extras: vllm
-            is_latest: true
           - cuda: 124
             cuda_version: 12.4.1
             python_version: "3.11"
             pytorch: 2.6.0
             axolotl_extras:
+            is_latest: true
     runs-on: axolotl-gpu-runner
     steps:
       - name: Checkout
@@ -87,12 +87,12 @@ jobs:
             python_version: "3.11"
             pytorch: 2.5.1
             axolotl_extras:
-            is_latest: true
           - cuda: 124
             cuda_version: 12.4.1
             python_version: "3.11"
             pytorch: 2.6.0
             axolotl_extras:
+            is_latest: true
     runs-on: axolotl-gpu-runner
     steps:
       - name: Checkout