Fix doc deployment by moving torch import

src/qibo/backends/pytorch.py

@@ -1,36 +1,24 @@
 """PyTorch backend."""
 
-from typing import Union
-
 import numpy as np
-import torch
 
 from qibo import __version__
 from qibo.backends.npmatrices import NumpyMatrices
 from qibo.backends.numpy import NumpyBackend
 
-torch_dtype_dict = {
-    "int": torch.int32,
-    "float": torch.float32,
-    "complex": torch.complex64,
-    "int32": torch.int32,
-    "int64": torch.int64,
-    "float32": torch.float32,
-    "float64": torch.float64,
-    "complex64": torch.complex64,
-    "complex128": torch.complex128,
-}
-
 
 class TorchMatrices(NumpyMatrices):
     """Matrix representation of every gate as a torch Tensor."""
 
     def __init__(self, dtype):
+        import torch  # pylint: disable=import-outside-toplevel
+
         super().__init__(dtype)
-        self.dtype = torch_dtype_dict[dtype]
+        self.torch = torch
+        self.dtype = dtype
 
     def _cast(self, x, dtype):
-        return torch.as_tensor(x, dtype=dtype)
+        return self.torch.as_tensor(x, dtype=dtype)
 
     def Unitary(self, u):
         return self._cast(u, dtype=self.dtype)
@@ -39,34 +27,41 @@ def Unitary(self, u):
 class PyTorchBackend(NumpyBackend):
     def __init__(self):
         super().__init__()
+        import torch  # pylint: disable=import-outside-toplevel
+
+        self.np = torch
 
         self.name = "pytorch"
         self.versions = {
             "qibo": __version__,
             "numpy": np.__version__,
-            "torch": torch.__version__,
+            "torch": self.np.__version__,
         }
 
+        self.dtype = self._torch_dtype(self.dtype)
         self.matrices = TorchMatrices(self.dtype)
-        self.device = torch.device("cuda:0" if torch.cuda.is_available() else "cpu")
+        self.device = self.np.device("cuda:0" if torch.cuda.is_available() else "cpu")
         self.nthreads = 0
-        self.np = torch
-        self.dtype = torch_dtype_dict[self.dtype]
         self.tensor_types = (self.np.Tensor, np.ndarray)
 
         # These functions in Torch works in a different way than numpy or have different names
-        self.np.transpose = torch.permute
+        self.np.transpose = self.np.permute
         self.np.expand_dims = self.np.unsqueeze
-        self.np.mod = torch.remainder
-        self.np.right_shift = torch.bitwise_right_shift
+        self.np.mod = self.np.remainder
+        self.np.right_shift = self.np.bitwise_right_shift
+
+    def _torch_dtype(self, dtype):
+        if dtype == "float":
+            dtype += "32"
+        return getattr(self.np, dtype)
 
     def set_device(self, device):  # pragma: no cover
         self.device = device
 
     def cast(
         self,
-        x: Union[torch.Tensor, list[torch.Tensor], np.ndarray, list[np.ndarray]],
-        dtype: Union[str, torch.dtype, np.dtype, type] = None,
+        x,
+        dtype=None,
         copy: bool = False,
     ):
         """Casts input as a Torch tensor of the specified dtype.
@@ -86,9 +81,9 @@ def cast(
         if dtype is None:
             dtype = self.dtype
         elif isinstance(dtype, type):
-            dtype = torch_dtype_dict[dtype.__name__]
-        elif not isinstance(dtype, torch.dtype):
-            dtype = torch_dtype_dict[str(dtype)]
+            dtype = self._torch_dtype(dtype.__name__)
+        elif not isinstance(dtype, self.np.dtype):
+            dtype = self._torch_dtype(str(dtype))
 
         if isinstance(x, self.np.Tensor):
             x = x.to(dtype)