[FEATURE] MXIndexedRecordIO: avoid re-build index (#20549)
[FEATURE] MXIndexedRecordIO: avoid re-build index (#20549)

When resetting, avoid re-build index for read-only instances to save time and
memory (in case of forked, memory pages can be shared)
szha/mxnet,tlby/mxnet,szha/mxnet,DickJC123/mxnet,tlby/mxnet,szha/mxnet,szha/mxnet,DickJC123/mxnet,tlby/mxnet,szha/mxnet,apache/incubator-mxnet,tlby/mxnet,apache/incubator-mxnet,tlby/mxnet,tlby/mxnet,szha/mxnet,szha/mxnet,DickJC123/mxnet,szha/mxnet,tlby/mxnet,apache/incubator-mxnet,szha/mxnet,tlby/mxnet,tlby/mxnet,apache/incubator-mxnet,DickJC123/mxnet,tlby/mxnet