[heterps]fix pipe-train bug: save and pull overlap may cause error

[danleifeng]

PR types

Bug fixes

PR changes

Others

Describe

fix pipe-train bug: save and pull overlap may cause get_field error
before:

Wed Nov 10 23:00:12 INFO: going to save delta model after update
I1110 23:00:29.506243 15886 ps_gpu_wrapper.cc:293] pull sparse from CpuPS into GpuPS cost 28.0899 seconds.
I1110 23:00:45.613412 15886 ps_gpu_wrapper.cc:425] GpuPs prepare for build hbm cost 16.1071 seconds.
I1110 23:00:46.988054 15886 ps_gpu_wrapper.cc:468] GpuPs build table total costs: 1.37453 s.
I1110 23:00:46.988124 15886 ps_gpu_wrapper.cc:536] thread BuildGPUTask end, cost time: 1.37462s
Wed Nov 10 23:01:16 INFO: begin save delta model for 20211106 - 3

after:

Tue Nov 16 08:51:46 INFO: going to save delta model after update
Tue Nov 16 08:52:10 INFO: begin save delta model for 20210815 - 1
going to save_delta_model
Tue Nov 16 08:55:47 INFO: ===========going to train day/pass x/2===========
I1116 08:55:47.992974 33335 ps_gpu_wrapper.cc:535] BuildPull start.
Tue Nov 16 08:55:47 INFO: cur_pass: 3 cur_path:
I1116 08:55:48.008092 33335 ps_gpu_wrapper.cc:293] pull sparse from CpuPS into GpuPS cost 0.015066 seconds.
I1116 08:55:48.038103 33335 ps_gpu_wrapper.cc:425] GpuPs prepare for build hbm cost 0.029945 seconds.
I1116 08:55:48.121467 33335 ps_gpu_wrapper.cc:468] GpuPs build table total costs: 0.083289 s.
I1116 08:55:48.121517 33335 ps_gpu_wrapper.cc:541] BuildPull + BuildGPUTask end, cost time: 0.128521s
I1116 08:55:48.121523 33335 ps_gpu_wrapper.cc:569] BeginPass end, cost time: 0.128571s

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