Shape changing bitcast and assert bitcast in disk (#3973)

* Shape changing bitcast

* only support it on disk

* basic test

* more tests

* RuntimeError instead of assert

* create unique temp files

* move tests that use disk to test_disk_tensor

* linter

* remove assert on error messages

* that's RuntimeError now

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Co-authored-by: George Hotz <72895+geohot@users.noreply.github.com>

tinygrad/runtime/ops_disk.py

@@ -53,7 +53,7 @@ class DiskRunner(JITRunner):
     # TODO: there shouldn't actually be casts here, bitcasts should fold into the load
     if ast.src[0].op == UnaryOps.CAST:
       top_src = ast.src[0].src[0]
-      # TODO: assert that this is bitcast
+      assert ast.src[0].arg[1], "disk only supports bitcasts, not normal casts"
       self.new_dtype = ast.src[0].arg[0]
     else:
       top_src = ast.src[0]