sample: Add output file option

Some dependencies erroneously print diagnostic
messages to stdout, mixing them with sampling result.

-file option allows workaround this

doc/flags.md

@@ -57,3 +57,4 @@ The sampling script `sample.lua` accepts the following command-line flags:
 - `-gpu`: The ID of the GPU to use (zero-indexed). Default is 0. Set this to -1 to run in CPU-only mode.
 - `-gpu_backend`: The GPU backend to use; either `cuda` or `opencl`. Default is `cuda`.
 - `-verbose`: By default just the sampled text is printed to the console. Set this to 1 to also print some diagnostic information.
+- `-output`: By default sampled text gets output to standard output, together with diagnostic messages. This option specifies the file to output the result.

sample.lua

@@ -13,6 +13,7 @@ cmd:option('-temperature', 1)
 cmd:option('-gpu', 0)
 cmd:option('-gpu_backend', 'cuda')
 cmd:option('-verbose', 0)
+cmd:option('-output', '-')
 local opt = cmd:parse(arg)
 
 
@@ -39,4 +40,11 @@ if opt.verbose == 1 then print(msg) end
 model:evaluate()
 
 local sample = model:sample(opt)
-print(sample)
+if opt.output == "-" then
+    print(sample)
+else
+    require 'io'
+    local outfile = io.open(opt.output, "w")
+    outfile:write(sample)
+    outfile:close()
+end