Default to Lua table if tds not installed

If tds is not installed because e.g. Torch with installed with torch-nv deb package then default to using a regular Lua table to store LMDB keys.
There is a limit of 2GB for heap memory in Luajit so we might not be able to train on some large datasets without tds.

tools/torch/data.lua

@@ -11,7 +11,7 @@ package.path = debug.getinfo(1, "S").source:match[[^@?(.*[\/])[^\/]-$]] .."?.lua
 require 'logmessage'
 local ffi = require 'ffi'
 
-local tds = check_require 'tds'
+local tdsIsInstalled, tds = pcall(function() return check_require 'tds' end)
 
 -- enable shared serialization to speed up Tensor passing between threads
 threads.Threads.serialization('threads.sharedserialize')
@@ -322,7 +322,14 @@ end
 
 -- Derived class method lmdb_getKeys
 function DBSource:lmdb_getKeys ()
-    local Keys = tds.Vec()
+    local Keys
+    if tdsIsInstalled then
+        -- use tds.Vec() to allocate memory outside of Lua heap
+        Keys = tds.Vec()
+    else
+        -- if tds is not installed, use regular table (and Lua memory allocator)
+        Keys = {}
+    end
     local i=0
     local key=nil
     for k,v in all_keys(self.lmdb_data.c,nil,self.lightningmdb.MDB_NEXT) do