Introduce forEach multi-stage domain specific language

[mratsim]

The goal is to remove reduceEach which was a workaround for Nim limitations in nim-lang/Nim#9490. This was fixed in nim-lang/Nim#9493.

The domain specific language should allow a "multi-stage" parallel section with a variadic for loop similar to:

proc reduction_localvar(s: seq[int]): int =
  omp_parallel:
    ### initialization
    var local_sum = 0

    ### for loop
    for i in `||`(0, s.len-1, "for"):
      local_sum += s[i]

    ### Finalization
    omp_critical:
      result += local_sum

Currently reduceEach require nb_chunks to be passed as params to omp_parallel_chunks (9ba351a) but this workaround was removed (dbd483c). Furthermore reduceEach requires allocation of a temporary seq while the DSL would leave partial_sums' tradeoffs at the user discretion:

• padding avoids false sharing and locks but requires temporary seq allocation which might be slow or restricted (embedded devices) and will require an extra pass on the temporary seq.
• OpenMP critical requires locking
• OpenMP atomic is only supported for a narrow subset of operations
• Nim builtin atomics

Parallel reduction on multiple tensors is used for vector dot product and all loss functions.

Also closes #3