[RPC] Terminate worker's childs first.

[cbalint13]

This small PR address the issue of orphaned sub-process in the RPC.

Description:
During autotune process on small remote edges (like rpi, rk3399, etc) the transfered tar is unpacked and further compiled into shared object. It can happen that sub-process (usually compile process) runs much longer and will get orphaned on timeout .terminate(). See the list below.

Problem:
Such longer running orphans overlaps with current measuring process using CPU cycles and even on worse case can lead to out of memory on limited 1G-4G low memory devices.

Fix:
The PR assures that in case of timeout all childen instances are terminated with parent too and with help of #3574 the residual files are now cleaned properly.

  ├─g++ -shared -fPIC -o /tmp/tmpy1javnqz/tmp_func_4ad0b36aa69fae99.tar.so /tmp/tmpy1javnqz/tmp_func_4ad0b36aa69fae99/devc.cc...
  │   └─cc1plus -quiet -D_GNU_SOURCE /tmp/tmpy1javnqz/tmp_func_4ad0b36aa69fae99/devc.cc -quiet -dumpbase devc.cc-ml
  ├─g++ -shared -fPIC -o /tmp/tmpy3cbmbck/tmp_func_34edb6ad7349ebf1.tar.so /tmp/tmpy3cbmbck/tmp_func_34edb6ad7349ebf1/devc.cc...
  │   └─cc1plus -quiet -D_GNU_SOURCE /tmp/tmpy3cbmbck/tmp_func_34edb6ad7349ebf1/devc.cc -quiet -dumpbase devc.cc-ml
  ├─g++ -shared -fPIC -o /tmp/tmp182f7df1/tmp_func_4888c9a4c281177d.tar.so /tmp/tmp182f7df1/tmp_func_4888c9a4c281177d/devc.cc...
  │   └─cc1plus -quiet -D_GNU_SOURCE /tmp/tmp182f7df1/tmp_func_4888c9a4c281177d/devc.cc -quiet -dumpbase devc.cc-ml
  ├─g++ -shared -fPIC -o /tmp/tmpjk358ic7/tmp_func_99d315bf5d46550.tar.so /tmp/tmpjk358ic7/tmp_func_99d315bf5d46550/devc.cc...
  │   └─cc1plus -quiet -D_GNU_SOURCE /tmp/tmpjk358ic7/tmp_func_99d315bf5d46550/devc.cc -quiet -dumpbase devc.cc-mli
  ├─rngd -f
  │   └─4*[{rngd}]
  ├─sshd -D -oCiphers=aes256-gcm@openssh.com,chacha20-poly1305@openssh.com,aes256-ctr,aes256-cbc,aes128-gcm@openssh.com,aes128-ctr,aes128-cbc
  │   ├─sshd                                                                                                                                  
  │   │   └─sshd                                                                                                                                ...
  │   │       └─bash
  │   │           └─python3 -m tvm.exec.rpc_server --tracker=192.168.1.2:9190 --key=rk3399
  │   │               └─python3 -m tvm.exec.rpc_server --tracker=192.168.1.2:9190 --key=rk3399
  │   │                   └─python3 -m tvm.exec.rpc_server --tracker=192.168.1.2:9190 --key=rk3399
  │   │                       └─g++ -shared -fPIC -o /tmp/tmps7iiv8bj/tmp_func_7e67258cb8079498.tar.so ...
  │   │                           └─cc1plus -quiet -D_GNU_SOURCE /tmp/tmps7iiv8bj/tmp_func_7e67258cb8079498/devc.cc -quiet -dumpbase devc.cc-ml

[cbalint13]

@eqy, @zhiics, @vinx13, @kevinthesun, @tqchen,
Please help with the review.

[cbalint13]

• Maybe related to [AUTOTVM] Cleanup subprocess when main process exit abnormally #3602 and Quit and clean when TVM is interrupted #3640 (but here we are on RPC side).

[tqchen]

Thanks @cbalint13 !