Fixes #583

Sycl USM array interface should use the base pointer in the 'data'
field, rather than its own 'data'.

```
In [1]: import numpy as np

In [2]: import dpctl, dpctl.tensor as dpt, dpctl.memory as dpm

In [3]: X = dpt.usm_ndarray(10, dtype="|f8")
   ...: Y = np.arange(X.size, dtype=X.dtype)
   ...: X.usm_data.copy_from_host(Y.view("|u1"))

In [4]: print("X =", dpm.as_usm_memory(X).copy_to_host().view(X.dtype))
   ...: print("X[0] =", dpm.as_usm_memory(X[0]).copy_to_host().view(X.dtype))
   ...: print("X[1] =", dpm.as_usm_memory(X[1]).copy_to_host().view(X.dtype))
   ...: print("X[2] =", dpm.as_usm_memory(X[2]).copy_to_host().view(X.dtype))
X = [0. 1. 2. 3. 4. 5. 6. 7. 8. 9.]
X[0] = [0.]
X[1] = [1.]
X[2] = [2.]
```

Author: oleksandr-pavlyk

dpctl/tensor/_usmarray.pyx

@@ -308,7 +308,7 @@ cdef class usm_ndarray:
         mem_ptr = <char *>(<size_t> ary_iface['data'][0])
         ary_ptr = <char *>(<size_t> self.data_)
         ro_flag = False if (self.flags_ & USM_ARRAY_WRITEABLE) else True
-        ary_iface['data'] = (<size_t> ary_ptr, ro_flag)
+        ary_iface['data'] = (<size_t> mem_ptr, ro_flag)
         ary_iface['shape'] = self.shape
         if (self.strides_):
             ary_iface['strides'] = _make_int_tuple(self.nd_, self.strides_)
@@ -335,7 +335,7 @@ cdef class usm_ndarray:
         """
         Gives the number of indices needed to address elements of this array.
         """
-        return int(self.nd_)
+        return self.nd_
 
     @property
     def usm_data(self):