As part of the Variable/Tensor merge, variable.tensor_data() should be removed in favor of variable.detach(). This PR removes tensor_data() call sites in Python Variable() and nn.Parameter() constructor paths.

Note that this PR is BC-breaking in the following way:

• For Python Variable() constructor:
  Previously, in-place updating a tensor after it's been used to create a Variable does not bump the Variable's version counter, which causes the following problem:

t = torch.ones(2, 3)
v = torch.autograd.Variable(t).requires_grad_()
y = v * v
t.add_(1)  # This bumps version counter of `t`
y.sum().backward()  # This computes `v`'s gradient incorrectly before this patch, and throws error after this patch

After this patch, in-place updating a tensor after it's been used to create a Variable will also bump the Variable's version counter, thus preserving the correctness of the Variable's version counter.

• For Python nn.Parameter() constructor:
  Previously, in-place updating a tensor after it's been used to create an nn.Parameter does not bump the nn.Parameter's version counter, which causes the following problem:

t = torch.ones(2, 3)
v = torch.nn.Parameter(t)
y = v * v
t.add_(1)  # This bumps version counter of `t`
y.sum().backward()  # This computes `v`'s gradient incorrectly before this patch, and throws error after this patch

After this patch, in-place updating a tensor after it's been used to create an nn.Parameter will also bump the nn.Parameter's version counter, thus preserving the correctness of the nn.Parameter's version counter.