For read access, there is .toCLong() (and if I recall correctly, also, .to<int64_t>()). Note that indexing to a 1-element tensor and then using this is inefficient.
For read/write access and CPU tensors, you can either assign to *(t.data<int64_t>()) or you can - before you make it a scalar - use auto a = t.accessor<int64_t, dim>() to get something that provides array-like access a[i].
Best regards
Thomas