Fix invalid cross-device and filesystem closed issues in Streaming ex…

…amples (intel-analytics#1656)

* Fix Invalid cross-device link in image_path_writer.py.
* Fix HDFS filesystem closed issue in ImagePathWriter and StreamingObjectDetection.
* Modify document.

scala/dllib/src/main/scala/com/intel/analytics/bigdl/dllib/common/zooUtils.scala

@@ -18,12 +18,12 @@ package com.intel.analytics.zoo.common
 
 import java.io._
 import java.nio.file.attribute.PosixFilePermissions
+import java.nio.file.{Path => JPath}
 
-import com.intel.analytics.bigdl.utils.File
 import org.apache.commons.io.filefilter.WildcardFileFilter
 import org.apache.hadoop.conf.Configuration
-import org.apache.hadoop.fs.{FileSystem, Path}
-import java.nio.file.{Path => JPath}
+import org.apache.hadoop.fs.{FSDataInputStream, FSDataOutputStream, FileSystem, Path}
+import org.apache.hadoop.io.IOUtils
 import org.apache.log4j.Logger
 
 import scala.collection.mutable.ArrayBuffer
@@ -94,7 +94,18 @@ private[zoo] object Utils {
    * @return Array[Byte]
    */
   def readBytes(path: String): Array[Byte] = {
-    File.readBytes(path)
+    var fs: FileSystem = null
+    var in: FSDataInputStream = null
+    try {
+      fs = getFileSystem(path)
+      in = fs.open(new Path(path))
+      val byteArrayOut = new ByteArrayOutputStream()
+      IOUtils.copyBytes(in, byteArrayOut, 1024, true)
+      byteArrayOut.toByteArray
+    } finally {
+      if (null != in) in.close()
+      if (null != fs) fs.close()
+    }
   }
 
   /**
@@ -132,7 +143,7 @@ private[zoo] object Utils {
    *                    get from cache (may shared with our connections)
    * @return hadoop.fs.FileSystem
    */
-  def getFileSystem(fileName: String, newInstance: Boolean = false): FileSystem = {
+  def getFileSystem(fileName: String, newInstance: Boolean = true): FileSystem = {
     if (newInstance) {
       FileSystem.newInstance(new Path(fileName).toUri, new Configuration())
     } else {
@@ -183,7 +194,16 @@ private[zoo] object Utils {
    * @param isOverwrite Overwrite exiting file or not
    */
   def saveBytes(bytes: Array[Byte], fileName: String, isOverwrite: Boolean = false): Unit = {
-    File.saveBytes(bytes, fileName, isOverwrite)
+    var fs: FileSystem = null
+    var out: FSDataOutputStream = null
+    try {
+      fs = getFileSystem(fileName)
+      out = fs.create(new Path(fileName), isOverwrite)
+      IOUtils.copyBytes(new ByteArrayInputStream(bytes), out, 1024, true)
+    } finally {
+      if (null != out) out.close()
+      if (null != fs) fs.close()
+    }
   }
 
   def logUsageErrorAndThrowException(errMessage: String, cause: Throwable = null): Unit = {