New allocator issue on big tensors (happens in benchmark and MNIST example)
While running the softmax benchmark:

https://github.com/mratsim/Arraymancer/blob/8d81857081724bc81625232b1457c06294541d62/benchmarks/implementation/softmax_cross_entropy_bench.nim#L165-L177

```
Traceback (most recent call last)
softmax_cross_entropy_bench.nim(341) softmax_cross_entropy_bench
softmax_cross_entropy_bench.nim(181) softmax_cross_entropy_backward1
init_cpu.nim(132)        zeros_like
init_cpu.nim(123)        zeros
data_structure.nim(104)  data=
assign.nim(155)          genericSeqAssign
assign.nim(118)          genericAssign
assign.nim(65)           genericAssignAux
mmdisp.nim(568)          nimNewSeqOfCap
gc.nim(492)              newObjNoInit
alloc.nim(738)           rawDealloc
alloc.nim(547)           freeBigChunk
alloc.nim(215)           addChunkToMatrix
SIGSEGV: Illegal storage access. (Attempt to read from nil?)
```