Wrap all Tensor factories

We need to wrap all tensor factories, add tests and docs for all of them. Currently only tch_randn is implemented and can serve as an example.

Here are the relevant lines of code:

C++ wraper: https://github.com/dfalbel/torch/blob/aa6742dc39ad903e89852bef8fd372af52392a37/src/tensor.cpp#L163
R wraper: https://github.com/dfalbel/torch/blob/aa6742dc39ad903e89852bef8fd372af52392a37/R/tensor.R#L114
Tests: https://github.com/dfalbel/torch/blob/aa6742dc39ad903e89852bef8fd372af52392a37/tests/testthat/test-tensor.R#L908
[x] tch_randn: Returns a tensor filled with values drawn from a unit normal distribution,
[x] tch_arange: Returns a tensor with a sequence of integers,
[x] tch_empty: Returns a tensor with uninitialized values,
[x] tch_eye: Returns an identity matrix,
[x] tch_full: Returns a tensor filled with a single value,
[x] tch_linspace: Returns a tensor with values linearly spaced in some interval,
[x] tch_logspace: Returns a tensor with values logarithmically spaced in some interval,
[x] tch_ones: Returns a tensor filled with all ones,
[x] tch_rand: Returns a tensor filled with values drawn from a uniform distribution on [0, 1).
[x] tch_randint: Returns a tensor with integers randomly drawn from an interval,
[x] tch_randperm: Returns a tensor filled with a random permutation of integers in some interval,
[x] tch_zeros: Returns a tensor filled with all zeros.

dfalbel / torch