[ready] perf: no noop cast just to make mypy happy (#1626)

Co-authored-by: Roelof van Dijk <roelof.van.dijk@vitestro.com>

tinygrad/tensor.py

@@ -52,8 +52,7 @@ class Tensor:
 
     # internal variables used for autograd graph construction
     self._ctx: Optional[Function] = None
-    if data.__class__ is LazyBuffer:
-      data = cast(LazyBuffer, data) # NOTE: this is a noop, it makes mypy happy
+    if isinstance(data, LazyBuffer):
       assert dtype is None or dtype == data.dtype, "dtype doesn't match, and casting isn't supported"
       self.lazydata = data if data.device == device else LazyBuffer.loadop(LoadOps.FROM, data.shape, data.dtype, device, src=data)
       return