Allow additional args to be passed many times (#1102)

This would allow
```
-a "scan_layers=false max_target_length=4096 use_iota_embed=true logits_dot_in_fp32=false"
```
to be
```
-a scan_layers=false 
-a max_target_length=4096 
-a use_iota_embed=true 
-a logits_dot_in_fp32=false
```

It will help avoid issues if quotes are not present during string
interpolation.

.github/container/test-maxtext.sh

@@ -13,7 +13,7 @@ usage() {
     echo "Usage: $0 [OPTIONS]"
     echo ""
     echo "  OPTIONS                    DESCRIPTION"
-    echo "  -a, --additional-args      Additional args to pass to MaxText/train.py"
+    echo "  -a, --additional-args      Additional args to pass to MaxText/train.py. Can be passed many times."
     echo "  --mem-fraction             Specify the percentage of memory to preallocate for XLA. Example: 0.90, 0.85, 0.65". Default to 0.90, contradicting JAX default of 0.75.
     echo "  --model-name               Specify the model names to run [Preferred]. If you specify model name then you do not need to specify decoder-block. Currently supported ootb models: 
                                        gemma-2b, gemma-7b, gpt3-175b, gpt3-22b, gpt3-52k, gpt3-6b, llama2-13b, llama2-70b, llama2-7b, llama3-70b, llama3-8b, mistral-7b, mixtral-8x7b" 
@@ -34,7 +34,7 @@ usage() {
                                        1. test-maxtext.sh -b 2 --model-name=gpt3-52k
                                        2. test-maxtext.sh -b 2 --model-name=gemma-2b --dtype=fp8
                                        3. test-maxtext.sh -n 1 -b 2 --model-name=llama2-7b --attn-type=cudnn_flash_te --remat-policy=minimal_flash --steps=10 --fsdp=8 --output train_output --multiprocess
-                                       4. test-maxtext.sh -n 1 -b 2 --model-name=llama2-7b --attn-type=cudnn_flash_te --remat-policy=minimal_flash --steps=10 --fsdp=8 --output train_output --multiprocess -a scan_layers=false max_target_length=4096 use_iota_embed=true logits_dot_in_fp32=false
+                                       4. test-maxtext.sh -n 1 -b 2 --model-name=llama2-7b --attn-type=cudnn_flash_te --remat-policy=minimal_flash --steps=10 --fsdp=8 --output train_output --multiprocess -a "scan_layers=false max_target_length=4096 use_iota_embed=true logits_dot_in_fp32=false"
                                        5. test-maxtext.sh -n 1 -b 2 --model-name=llama2-7b --attn-type=cudnn_flash_te --remat-policy=minimal_flash --dtype=fp8 --steps=10 --fsdp=8 --output train_output --multiprocess
                                        6. test-maxtext.sh -n 8 -b 2 --model-name=llama2-7b --attn-type=cudnn_flash_te --remat-policy=minimal_flash --steps=10 --output train_output --fsdp=8 --data-parallel=8 --multiprocess
                                        7. test-maxtext.sh -n 8 -b 2 --model-name=llama2-7b --attn-type=cudnn_flash_te --remat-policy=minimal_flash --steps=10 --output train_output --fsdp=4 --tensor-parallel=2 --data-parallel=8 --multiprocess
@@ -76,7 +76,7 @@ eval set -- "$args"
 while [ : ]; do
     case "$1" in
     -a | --additional-args)
-        ADDITIONAL_ARGS="$2"
+        ADDITIONAL_ARGS="$ADDITIONAL_ARGS $2"
         shift 2
         ;;
     --mem-fraction)